I used passportvisasexpress.com. I didn't have any issues with them; the only annoying thing was that they didn't call me or email me on Tuesday to tell me my wife's visa had been approved. But other than that, they were an excellent company. I had mine expedited with a two day turn around, and they only charged me $50 dollars for my wife's, instead of the 125, because it was a dependent visa. they were very thorough and clear about what I needed to send in, and the on the whole the process went quite smoothly. So much so that I've already purchased my plane tickets for Glasgow, and now I just have to pack.
