Well, i'm personally a very big fan of Teagarden. Probably the best thing to know about him is that he is very good defensively, so there will never be worries about him moving out from behind the plate. Another thing that should speak about his abilities is that Texas does not want to move him unless they get a premium pitching prospect in return.
As far as projecting what he is going to do next season and beyond is a little more difficult. He obviously has power, and i'd say if he has 400 ABs next season you can expect 20 HRs as a baseline. The average is a bit harder to predict...he has hit around .300 in most of his minor league seasons, but last year he seemed to fall apart in the minors. Of course he came up to the majors and turned the baseball into mashed potatoes. Can he carry the average and power into the majors next season? I'd say a .285 average would be solid prediction from him.
As far as career, if he puts it together with the bat he can be a .290-.300 average with 25-35 home runs annually. Obviously playing your home games in Texas is going to help your HR totals. Only thing i'd be worried about is him not starting behind the dish. The main competitor for him is Saltalamacchia, and many people feel that one of them will win the job in ST and the other will go back to AAA to start the season. I'm really praying that Salty gets traded so both can start and begin putting their careers together.