Whether Sainsmart can make good stuff is not the question. If you're buying a board for which someone else has developed a reference hardware and software package then there are fewer opportunities for issues to arise. After all, the Arduino team enables the IDE and publishes the hardware specification.
Where vendors like Sainsmart can fall down flat is new hardware that someone developed for them but which is largely undocumented, for which there are no easy-to-use libraries, etc. They're not alone, BTW, even Sparkfun will copy someone else's hardware (USB host controller shield, for example) but not offer working software (because they want their users to copy Oleg Mazurovs work and then adapt it for their own use).
This is where Adafruit is different. They offer lots of internally developed libraries and sketches to support their user base in developing their own work.
If you're buying a board for which someone else has developed a reference hardware and software package then there are fewer opportunities for issues to arise.
I have just ordered a Sainsmart 2560 & touch screen tft display combo. I have read a few topics on here about them but the number of problems raised compared to the number sold must mean that either a lot of people are happy with their purchase or couldn't actually get it to work but were too embarassed to say so. (We've all been there!)Would anyone care to share their experience?