I am a POSitouch tech/programmer and POSi is about as strong as any out there. There are literally tens of thousands of POSi installations throughout N America. I also support Dinerware and POSi has a steeper learning curve but the functionality is outstanding comparitively; Dinerware is great for 1-2 terminal sites looking for a simpler setup. POSi can be utilized in a greater number of configurations and is used by enterprise level customers like Cheesecake Factory, Outback, Dairy Queen, Fuddruckers, Carrabba's, etc.
I do agree with DrZogg in that the quality of support is perhaps the most critical part of choosing a POS system. The dealer and their support team is often the most important aspect. Having said that, there is definitely some subpar POS software out there. I have nothing bad to say about Aloha and think it's fairly comparable to POSi...but like I said, support should be a HUGE part of your decision.
Let me know if I can help.