The staff at Baseball Prospectus took a look at Vincent Velasquez and Mark Appel recently and turned in some good reports.
For Velasquez they noted that he touched 96 with his fastball while sitting more comfortably at 92-94 with the makings of an average Major League curve. BP projects him as a mid-rotation starter, though his velocity could play up better as a relief arm.
With Appel, they noted that his first three fastballs the night they were watching were 96, 97, and 98 mph, though he settled in at the same 92-94 as Velasquez with good command and some arm-side run that gave right handed hitters a hard time. He didn't throw any changeups during BP's look at him, but they noted that his slider should be a very good pitch for him at the major league level. They also said that he's almost major-league ready, has the "raw tools to be a monster on the mound," and profiles as a number 2 starter, depending on if he can pitch with "that extra edge."
There are some good notes on the mechanics of each, as well.