I've owned two and sold two. If your car is pretty much stock and running properly, the power tune may be of some help. The other features will certainly be beneficial. The HPP3 is the least expensive of the most popular handhelds and also has the least flexibility for tuning. The other guys have the abilility to tune out DTCs for running catless and to fine tune, PE, fuel trims and such. I still have a Predator but it too has lost relevancy due to different heads and cam. As I experimented with shift points and firmness (automatic), eliminated CAGS (6 spd) and changed tire diameter the Hypertech was useful and each time it was used it became more cost effective. When I wanted to go to the next step, the sale of these tangible tools made them an investment. In other words, I spent very little on the experience because I recovered nearly all my costs. The Hypertech was rather fun and for someone new at the game it had its place and certainly served a purpose.
If you make any major change to your powertrain you will not want to use the HPP3. That is when tuning turns critical. A professional reading your exhaust while measuring the car's power on a dyno is how it is done and has no equal unless you spend a thou and learn to tune it yourself.
If your car hasn't much more than a CAI and catback and you're not looking to get the most out of your engine then go for it, especially if it is used (and unlocked). The HPP3 is a bit of a high tech toy but a useful toy. Unlike a session on the dyno or a mail order tune, at the end of the day it can be sold again.