Of course in order to get a guaranteed trip against someone, you'd need to have a +20 bonus to trip attacks. And that's if your opponent has no bonuses whatsoever. And you still need to hit with an attack. If you fail, you're risking a retaliation.
Most of the time, trip's going to come in at a 50/50 proposition. Let's look at an extreme case anyway, just to be sure.
With imp trip, a trip weapon (+2 to trip attempts) and a strength of 20, you've got a +11 to trip attempts. Assuming your opponents strength AND dex are both 10 (which isn't particularly common), you've got an 88% chance of success after you hit. Assuming a first level fighter, you now have a +10 on your attacks, both the improved trip attack, and the AoO for your opponent standing up. On a creature with an ac of 10, you've basically done
(80%*88%*(95%+95%))/85%=167% of a normal rounds worth of damage.
Bring this into more normal sort of regions, up against a real front-line creature. Typically you'll be looking at a +2 from strength on that (which drops your chance of a trip after a touch attack down to 75%). The ac you're up against will also be around 15 or so.
Damage for the round becomes
(80%*75%*(80%+80%))/55%=175%
Note that this is against a creature with no dex bonus to AC at all.
If the creature instead has +2 from dex (but the same overall ac), you do 134%.
If it's got a +5 from dex, you're down to 75% of normal damage.
Also this all assumes that the target stands up in a threatened area. If he doesn't, improved trip would almost always result in you doing significantly less damage to a foe than just hitting them.
Personally, I would have liked to see trip follow the same mechanics as most of the other opposed combat moves, and include the BAB of the combatants (not to mention incorporating the balance skill somehow).