I think the acheivement system is great in some cases. In my experience, most achievements are simply ways to force the player to play *longer* without having to do things that require much skill, or that require the player to do anything not required by the game anyway.
For example:
A lot of the trophies in Rock Band are simply tasks like "get a tour bus", "get a plane", etc...which are tasks that will just happen as you play the game. Is a trophy really necessary?
In some cases, the trophies are really cool extra things to work towards, such as, again in Rock Band, reaching 1 million points in a song, achieving an 8x band multiplier, etc. In the majority of cases though, it takes little more than pure grunt work to reach the trophy and I think that's just poor design on the trophy creators' part. I love the concept, it just seems like a lot of them aren't very interesting. The ones that are add a lot to the game. The ones that are just grunt work to achieve don't add much in my mind.