From an ebay buying guide.
http://www.ebay.com/gds/Buying-Grade...1342856/g.html
The main caution regarding BGS is that they will grade sheet-cut cards. Somewhat controversial in the hobby, some collectors won't touch sheet-cut cards, while others have no problem with them. Your own personal stance will help to determine whether you will consider BGS when shopping for graded cards. BVG is Beckett's vintage grading department, and the same caveat regarding sheet cuts applies. BVG recently stopped providing subgrades, which was the easiest way to pick out a sheet-cut card (typically, the Surface subgrade on a sheet-cut card will be lower than the other subgrades), so be careful if this is a concern for you.
Google Beckett or BVG and sheet cut and you will see lots of discussion of this issue over the years.