I have, and sadly, due to the fact that they now pop like Undead and Daemons, they suck unless you have ALOT of them in one unit.
Swarms in 6th ed were extremely overpowered, they could pretty much hold anything in place for a long time. Now GW has made them all crumble like undead to balance them a bit, the rule went a touch far though. All the books that haven't been updated yet have very expensive swarms that are worthless, the updated books are seeing a drop in points to compensate for their crumbling. However, it hasn't really been enough to tempt people back to the little buggers. They still seem expensive for what they do and you are better off spending your points on other things. I can't remember their exact cost, but if it is around what I think it is you can easily get 3 saurus for the cost of one swarm, or a unit of 15 saurus for the cost of a unit of 5 swarms. This will actually count as a core choice and could potentially hold a unit in place for as long as you would expect some swarms to given their crumbling, but it would also deal some serious hurt back.
In 6th ed I would always take 6 swarm bases. Just identify the opponent's hardest unit and send the swarms over to tie them up for the entire game! Now I don't think they're as good, since 7th ed means they crumble like undead. Dunno what they're like in new book as I haven't actually read it yet!
Against lightly armored high toughness enemies they are cool - I often use 2-3 swarms, Giants, Minotaurs, Ogres etc - they hate the Auto wound from poison, and with a Higher than average M you can often get the charge off as well. Giants hate Skinks and Swarms I normally move up, fire 10 - 20 blow pipes, and then charge with the swarms to Kill the giant off. Quick, easy and relatively cheap.
I use them as 1 base units, just to put of advances and hit something with high toughness for one attack. But i do tend to take them less often now.
I would take 3 and i would line them up from your deployment zone the the nearest frenzied unit. LOL have the unit kill it and over run into the middle one. On your turn he kills yours and on his turn he frenzy charges the last one and then runs off the table. LOL but then again i have not see it work yet because i dont run them.
They are listed as unbreakable swarms. The BRB explains that they take wounds equal to the amount by which they lost combat.
I had a unit of 2 flank charge 4 trolls and deal 3 wounds. I had CR 4, he had outnumber, he lost and fled through his whole line and never rallied. I haven't taken 'em since.
Back in the 6th edition, when someone actually used swarms, they ruled. Now they are probably the worst unit-type in the game. I remember having 10 bases of snotlings in almost every game, that's a hella-wide-unbreakable-screen-unit. It was too easy to hide the most important parts of my army behind the stunties, usually trolls and black orcs were the ones to stalk behind their smaller friends. Nowdays snotlings are not 'swarms' and you can take as many of 'em as you want, but they have lost the unbreakable rule, and got stubborn + immune to psychology. Well I wouln't mind unless they had Ld4, which they do. So they don't crumble, but dude, the swarm breaks! That is not cool, NOT COOL! The only thing they can be used today is in a goblin army as a frontal force, honoring the tradition most sacred to the O&G: stunties first, den da boyz (in this case da boyz being goblins being bigger than snotlings). Well enough about snotlings...when playing lizardmen, don't take swarms, cause they suck. Use skinks instead, they rock.
I have only played a couple of games, and only used swarms in one of them. However, in that one game, they were worth the points. I had 3 bases of jungle swarms together, and sent them into a unit of 3-4(can't remember which) ironguts and a butcher. I expected them to be a distraction, hold his stuff back for a turn. They actually killed an iron gut, won combat, and held for a second turn before finally meeting their end. They are also good for using as a screen for your razordons/salamanders (I had a second unit of swarms. Same battle) and they won't block line of sight do to their "small" rule.
IMO swarms can be used (but not reliable) as Trappers... The main problem is the number of base in swarm unit... if swarms charge and if they arent destroyed in the first round of combat, the other units can take position for flank charge on their next turn... it is like as playing with 40k Tyranids (if you destroy enemy in the first combat round the other enemies shoot you down, but if you destroy enemy in his round then you can charge and probably destroy another enemy unit) but more sacrifacial way...(usually you cant destroy enemy with swarms and you want enemy to destroy them in his round...) I generally use 2 unit of swarms (each unit has 2 base in it) ... I use them both in the same flank... generally one of them ties my enemy's flanker unit and other one passes through enemy lines and charges enemy's long range supports ( empire hand gunners, dwarf thunderers, war machines)... once 2 of these units destroy 10 dwarf thunderer and an organ gun...
I agree, Terradons are great for droping rocks on enemy fast cavalry... but they are not as good warmachine hunters as before in the 6th edition... 3 model terradon unit can be beaten by even imperial warmachine crew... I dont say anything about High elf(ASF) or Dwarven(stubborn, T4, 4+AS) crews... but 2 base of swarms can ties them for 3-4 combat round even if dont destoy them...
3 terradons attack let's say high elf bolt thrower crew (2 guys). High elves strike first with 2 attacks hitting with 3+. Most probably they will hit with both. They wound on a 4+, so they inflict one wound to the terradons, luckyly they have 2 wounds so none of them is dead (mustn't forget the 6+ armoursave). Terradons strike back with 3 attacks and hit with 4+. Depending on how they hit they have 4+ chance to kill one or two crewmen. Even if they don't kill any, they still get a draw with 1 wound against overpower. Standstill. Also when compared to jungle swarms, terradons have a LITTLE bigger chance of getting into close combat with artillery crew. Oh and dwarf crews have an AS of 6+.
I think they are awesome STILL. Yeah they were overpowered in the last edition but I found a way to exploit their lost cost. This is what I do... In my list I take 4 one base units. Keeping them at one base opens up many options during the game. For example, warmachine hunting, the ever-annoying march blocking, chasing already fleeing units, and speed bumping. I mainly use them as "speed bumps." With this tactic I'll send one unit up an inch away from a big expensive unit. On my enemy's turn, he'll be forced to charge it, beat it, and overrun. When he does so he'll run into another one base swarm 2-3 inches in his overrun. Finally, I'll charge in my Steg and other Big units on my turn. Try it! It is fuuuun.
Unless he restrains the overrun, or in some cases the opponent just won't charge the swarm and choose patience to make sure his unit doesn't get put in a bad position. That kind of baiting tactic is what I do with HE eagles, and sometimes it works great but other times it just loses me the eagle. It is a bit risky, and in your case you are using two swarms to pull this off. If you get it working for you though then great! Don't let people convince you not to take them, but I know that none will ever find their way into my army, and most people are the same.
Yes well there are always other units that can perform other roles on the battlefield. I have used them in a few battles now and have had great success with them and now they have a bit of a reputation against my opponents (coupled with their ideas of what they were like last edition) so far they have earned their points back every game and most times more than their points. I like to use them for variety in my army composition and for the feel and theme they bring to games. Ultimately, it's up to you, and your playing style and army selection. Just don't decide you won't take them until you give them a go. They still do alright despite the crumbling rule, you just can't be as reckless as last edition with them.... Make your own mind up but as I said I always believe it is best to try everything three times before making a final decision on them... they're in the army list for a reason....