I use Blackmagic Ultra studio Pro and it is worlds better than Matrox. I have tried Matrox Rt X2 and Matrox mini with max and both had lots of problems and slowed the machine down. The last time Matrox worked well for me was with CS4 + Matrox RT X2, since then nothing but trouble.
I had a BM Infinity Pro and was satisfied with it until CS4. The CS5.5 drivers did randomly crash, especially in conjunction with Mercury Play Engine and H264 footage. The BM-support recommended to better switch it off - Thank you BM ! The biggest bad point for me was the delay of 1-2 sec of fader movements in PPro's mixer window. This was really unusable for mixing sound. All updates didn't bring any fixes for Intensity Pro, maybe this is better with Ultra studio, but I've lost my faith in BM's programmers and switched to a Matrox MXO2 mini.
I tried an ordinary mini without this MAX-option which is definitely only useful for H264 exports on systems with slower CPUs. The drivers are even worse than BM's and I was very disappointed - until I tried the actual beta drivers 6.1. With them the Matrox device is running stable and much faster than my Intensity Pro. I noticed that the graphics card (quadro 2000) is used much more than in BM timelines so I think the Matrox engineers understand the Adobe architecture much better.
I agree with Spreeni, the new 6.1 beta drivers are a revelation. Premiere now just works, there are no random freezes or hiccups any more. The range of analogue in/outs provided by the MXO2 mini are invaluable - you need to go further up the range for professional in/out.