I have asked the same question of a few people (and from personal experience!) and the answer is almost always the same - yeah use it but keep an eye on it!
They do tend to do their own thing sometimes, a friend and I did a night flight back from Scotsdale, Arizona across the desert to Longbeach, California - not IMC but with no visual references it may as well be (if you have never done a lfight across a desert everyone should do - completely black! strange!!). We were using the AP on the long legs when we realise that it had gone into a shallow descent. The AP was doing nothing to correct this, we ended up disconnecting and retrimming the aircraft - after that we kept a close eye on it.