I think mobipocket is fine for creating mobi's but I never use it to preview. In fact I prefer it to calibre for creating mobi's (although I use calibre to turn mobi's into epubs).
Typically for testing I use:
Kindle previewer
Kindle for PC
B&N for PC
Sony reader for pc (although using it less and less)
An actual Kindle
kindle for iPad
kindle for iPhone
iBooks
And then when I'm finished, I upload and get all paranoid that maybe I didn't test it enough.