Brett - I am using an SP C1 mic, and at least to my ears, the VTB-1 definitely warms it up. The tube blend is wonderful, the high-pass filter is very useful, and the choice of either a Hi-Z 1/4" line in (say, from a bass guitar) or an XLR in (with switchable 48v phantom power) is great. The unit has my high recommendation. My only qualm is that it is a single channel unit. But if you are doing stereo recording then you can just buy two. The price makes that an easy reality.
Also, if you are thinking about buying one, I recommend humbuckermusic.com - as they had the best price on the unit that I was able to find.
I picked up a FMR Real Nice Compressor (RNC1773) while I was at it. If you need a compressor, it's an awesome unit. I use a TRS (tip-ring-sleeve) cable to run it though the insert jack on the VTB-1. I must say that the combo those two make is unbeatable for the price. $300 bought em both.
-Allen