I've got one on my 400D and its never come off since I bought it - a fantastic addition to a DSLR for so many reasons - including:
1) I don't have big hands, but even I found my pinky finger would fall off the bottom of the camera when using it; which led to it rubbing the edge which just gets irritating. The battery grip solves this by giving the pinky finger somewhere to hold onto.
2) 2 batteries at once in the camera gives a great battery lifespan - though I would say always keep at least one spare battery outside of the camera (you can use 1 battery in the battery grip if you only have 2 in total). This is because I find that the grip power report will drop from half to low and then to nothing very fast, but will sit at half for a long time; so it can catch you out. I keep 4 batteries - two in and two out so I've always got at least one to swap in when those in the grip give out
3) The vertical controls and hand grip give a great support when shooting; plus whilst some say "you only need portrait if you shoot portraits" I've found that any field of photgoraphy you'll end up taking landscape and portrait aspect shots - so having those extra controls and the grip really makes it a lot easier
4) I can use a canon wrist strap with the battery grip - a great addition in its own right for helping to hold the camera so another plus for the battery grip (you can get them which clip to your tripod mount instead, but they always look a little fiddly to me for actual use)
5) Weight - overall this might be less of a factor if you use lighter, smaller, lenses but the added weight does help a little with balance in the medium weight lenses.
Overall the grip is all positive things for me (save actually paying for it

). There are only three things to look out for
1) Don't forget that it has a battery door holder to put the battery door from your DSLR in for safe keeping (a fantastic idea!)
2) Jiggling - sometimes the contacts in the grip won't make a perfect connection to your camera body - just take the grip off and put it on again. Generally a bit of jiggling and it goes away and once it has a solid connection you should never have any problems thereafter
3) Over tightening - if you over tighten the screw mount the grip can become stuck onto your DSLR - this has happened to mine, but honestly since I've had it I've not had any reason to want to take it off (and even if I could the strap strap loss and having to fiddle with it would be a pain each time).