It should be noted that GoSmile is chemically different than most whitening systems. It is specifically designed to whiten without causing painful sensitivity.
As someone with extremely sensitive teeth, I can tell you that I found GoSmile to not be painful in the least. It did irritate my gums but I used it at home and I may have just been too aggressive at the gumline.
That said, because it is different (peroxide based) some people may not see the optic whiteness of other systems. This is more of a gradual, natural whitening.
I think GoSmile is a great product and it is the only way that I can even think about whitening my teeth.
However, if you are not prone to sensitivity or are looking for bright, optic white teeth, I would use Crest Whitestrips.
Either way, I'm not sure I would have it done onboard. Not because of the pain issue as others have mentioned (there is none) but because you can buy the whole GoSmile system for around $80 and have it done at home. Of course if you want to spend the extra $$ to have the spa experience with it (whatever that is!) then.....go for it!!