They are very worth it, except when they aren't.
Mostly, the tier builds are just about giving you extra options for how to build your army, rather than giving you any sort of boost, and indeed most of them have more weaknesses because of their restrictions than strengths due to the bonuses. However, some tier lists are quite competitive, and very fun to play. I usually wouldn't recommend aiming for a tier list until you had enough models for a general mix-n-match army, especially if your a newer player. That said, if you only have to swap out a couple of models to make your general list into a tier list, then I say go for it.
Ok i will post up my new list idea's that match there tier...in my 35pt Thread, please let me know what you think im not sure which to go with!