Trifecta was the first available tune for the Sonic, and since BNR got on board with the forum as a vendor, people naturally gravitated that way. Service has slowed substantially, and people have reported problems with BNR and Trifecta for that reason... but as far as I know nobody has complained about the product itself, service aside people have been overwhelmingly positive about Trifecta.
Other tunes I have heard of are Vermont Tuning, HPTuners, and something about a chip.
I have had both Trifecta and VT and am very satisfied with both, while at the same time, still curious about the others, and about getting a custom tune done at a dyno shop. Tunes seem to be the most powerful and effective change you can do to your Sonic, the biggest bump and required in order to get any additional performance out of almost any other mod.