mralx, no, I have not tried one. I have had a vehicle tuned before, and it was never under $250-300 for a good chip/tune. And on my aveo when I had one I had a CAI (with an AEM bypass valve) and a catless header and I did feel noticeable gains in my top end, although getting tuned for them would definitely unlock more gains.
As far as the eBay chip online - it's very risky. IIRC, you could get your computer (ECU) fried. But if you're willing (with lifetime warranty on it and why don't buy the chip ($10), run a baseline dyno (around $60-100), then install the chip, and run another dyno to see the gains and report back on here what you've gained. That would be actually a useful post.
Thanks ahead and looking forward to seeing what gains you will get from a $10 mod.