They are decent, if you are in a static 4 man group they are a great tank since you have a good AOE ability, high HP, self-heals, and high DPS. If you are playing by yourself don't bother, end game dungeons are miserable in pugs and you will want to go play in traffic instead of running dungeon finder after a day or two - there is literally only a single dungeon really worth running at 50 (AK). That said, tank queues are instant all the way from 1-50.
For really high end stuff the big problem is that paladins have better damage reduction (but less HP) and that warriors don't have a quick interrupt or even taunt (but you can get it from leveling glad to 22 at least, no such luck with interrupt). Warriors (marauder 30+ job) can tank everything fine nonetheless I believe, you'll have to ask a raiding healer which tank they prefer to work with. Unlike other MMOs there really isn't a vast set of dungeons to farm (yet?).