I've been very impressed with everything about twinBASIC so far. There is a free (including commercial use and royalty free distribution) community edition (Windows only, 32bit and 64bit - though the 64bit has a nag screen).
The paid editions add optimised compilation, and (planned) support for Linux, Mac and Android (and possibly Web).
Compared against other products such as Gambas, Xojo (realbasic) or VB.Net it has the advantage of being able to import existing VB6 apps and run them as Windows 32bit apps (much of this is available now, but there is still some way to go for 100% compatibility). RAD Basic may offer compatibility too, but it seems difficult to find an up to date evaluation download.
twinBASIC also goes beyond what VB6 offered - 64bit compilation, multithreading, creating DLLs etc. (some of which could be done with hacks in VB6).
As for a potential market, that is difficult to say. There are still huge numbers of VB6 applications in use, having the ability to move easily to a new language may be attractive for whoever still supports those applications. There is certainly interest, a thread about twinBASIC on VBForums has had almost 400,000 views.