PSP do everything with character and quality. Mixsat is no exception - the bundle is good enough to be used on mastering, although Mixsat was intended more for mixing than mastering - Vintage Warmer is their big brother for mastering. Mixsat is single band as opposed to the 3 on offer in VW, but if your final mix is a good one, I find Mixsat good enough for mastering.
It imparts a very ear-pleasing warmth to a mix or to any group. The compression on the highs is intended to add top end sparkle, but personally I find it a little artificial-sounding and steer clear of it. But with a good mix, you don't need it anyway. The valve sim however makes any mix fuller and "warmer" (which is a pretty subjective term I suppose). Because of the way it emulates valves, you get limiting with harmonics - and you can choose 3 types of valve - which basically adds in higher order harmonics for the 3 types. I like the PSP sound of their valve sim - even on high settings it still sounds musical, and on type 3 it may hurt a full mix, but it is invaluable for crunching up a drum group or individual loop. You don't have to worry about compressor settings, attacks, releases, thresholds etc - you just turn up the valve% until you like what you hear. It's the equivalent of having Eq, compressors, a limiter and an enhancer all in one easy interface.
If you want big bass, then Mixsat will deliver - can make weedy basslines sound full and thick or just add that extra little sub - to my ears better used in the mixing stage rather than clamped over a full mix, but some people will like that effect too.
It really is a studio essential - useable in channel recording or mixing stage, group mixing or mastering. It doesn't hog too much CPU (I get around 10% or under with all features turned on - Athlon1.1GHz, 512Meg RAM) Which considering the great effect it gives is very good.
Presets - comes with a useful set. From preamps to drumloops, to digitalising to valving to mastering, and bassifying or sparklifying. Not too overdone mostly as some plugin presets tend to do. You could actually make do with presets only and never have to tweak it?
Interface - it's very obvious what's going on - no rocket science needed at all. No manual needed.
VFM - alot depends on the kind of sound you personally like. I feel it's one of the best valve sims out there and I consider it much better value than enhancers. To get an equivalent sound you would need a good Eq, an enhancer and a good comp or limiter (not that you shouldn't have those anyway!). Some people may not like its sound - it certainly adds its own character and plenty of it, so in some ears it actually ruin their sound.
One small reservation - it does tape saturation sim also. If you like this effect, you might need to check out the competition - Personally I find T-Racks does a better Tapesat, as does Endorphin, but that's not why I have Mixsat - it's the valve sim that shines. But its tapesat is still very useable.