My personal experience with 40man block so far : Needs to be practiced. As a new player I responded pretty well into playing the game generall, untill the time I used a 40man block. It must be my biggest failure as a new strategist so far. Bad placement / made my general not fight for 2 rounds due to block and got tied up badly without really making it worth it although I invested quite a LOT on it : Rerollable all hits, rerollable all saves, Sunclaw starhost. Did a hell lot of wounds but in the long run I am 200% positive that I only utilized about 50% of the units' strength
Another comment : our 5+ save is bad. Wish it was 4+ without ignore rend but since it is not the case : Next time I am trying this with a mystic shield on. While I 'm gonna miss it to some really important unit of ours, 5+ even rerollable due to priest, is just bad. + the -2 rend gives no saves to our horde while the 4+ without rerolls can still have a chance to save some of their models. So this is definitely something to keep in mind in my opinion.
By the way I would like the opinion of the most experienced players on this. Wanted to debate this to my battlerep but as I am pressured these days it will take me sometime before I do so, so I ll just pose the question now !