As I've said in other threads, Mongoose has a VERY poor reputation (at least around here) with their d20 products. I would never allow any Mongoose books at my table.
I've seen some rather nice ideas in mongoose's books, however I wouldn't let them in as is in most cases. While they can be great inspiration the balance is usually off for my group. However that's not to say it can't work for your table just keep an eye on what people are choosing and why.
I must say usually the biggest problems come when you mix 'splat' material from two different sources... something mix too well and others not as well. The problem usually comes from the fact the two companies can't directly address potential problems due to copyright issues.
What do you guys think of using the spells that are in the Ultimate Divine and Arcane books from Mongoose? Would that be worse than using wotc's Spell Compendium hehe?
I would agree that mongoose products are not particularly well designed with balance in mind for all levels. I haven't looked at those specific books but as a general rule, we barred Mongoose products from our games. If you like ultra high powered games based on character abilities, I'd say go for it, it simply doesn't work for my group.
I've seen some rather nice ideas in mongoose's books, however I wouldn't let them in as is in most cases.
I agree with Abraham here. Some of their stuff is quite inspirational and if you're willing to tinker (usually power down, but not always) with what's presented then it's suitable IMO.