In my experience thus far and I work with many engines, FPSC is capable of competing with most of them in terms of overall game creation capability. You can as with any engine - within the limits that they allow create good games with it, certainly equal to most indie engines and it can be a lot easier using its in built features to advantage. If you want to stray a little from what its designed specifically to do and make your own content and push the engine to the limits of its level design capability then you may even make even better games but it will take more time, quite a lot more.
There is some room for future improvement and currently the AI and FPS needs improving but no one yet knows how that will pan out.
FPS I can say is not great but thus far playable when it comes to rather complex scenarios. I am using a great deal of in level entities though most are static of course and dont draw so heavily on CPU time so I can have a large number.
I have not found any great deal of FPS improvement in a final .exe over that of the editor in test mode - though there is a small improvement and gameplay smoothness can be slightly improved.
AI I cannot definatively say as I have not yet work extensively with it to see what can fully be achieved with a combination of careful level design, Patfinding and and AI scripting (that yet to come shortly) - has anyone?
The default enemy characters behaviuor if you choose to use the correct AI scripts is enough to allow enemies to behave in a reasonable behavioural manner and additionally kill you if you want them to. Its probably true to say though that the default enemies and their associated scripts need some improving so that those downloading the software would have no reason to complain from day one.
Any engine will never be as good as we might like, as humans we are never satisfied. Except for FPSC currently though, I dont see may others even allowing users to complete many games.