She becomes quite explosive late-game once you get about 2 1/2 DPS items. You can melt DPS champions in 4-6 shots (literally 1-3 seconds with Rapid Fire), and solo-kill turrets in one go of Rapid Fire with just a few creeps as support. She's so overpowering, when I first read the AP Tristana threads I initially thought they were just trolls.
Early/mid-game she's great at harassing other heroes in her lane. Her long range means you can pretty much poke other heroes whenever you want, and heavily deny their last-hit opportunities. My life-goal is to force someone in my lane to quit before level 8, just from harassing them with single shots.
The problem is she's the squishiest hero I've ever seen. Any player that's any good should primary you as soon as you start auto-attacking, forcing you to run. And if she's not standing and shooting, she's next to worthless.
Even worse, for the team-fight environment we're playing in now, her AoE damage is super-weak. I call BS on the guy that said he uses her in tourney and high-level play. It's currently a team-fight-centered metagame, period, and I expect Tristana to either die early in any high-level teamfight, or not deal enough damage to be viable.