bapu
The Waves API2500 is pretty slick. After using it I decided to NOT even try the UAD version.

Just demoed the API2500 Waves version and I agree with Bapu and Jeff.
 
It works really well on the master buss as a "glue" compressor.
The drums pop right through and the instrumentation is brought up to fill in the gaps, without excessive pumping.
It has a switch to turn off the analog harmonics generator and it works. I checked it with a test tone and spectrum analyzer and it is clean with the switch off.
 
The PSP used in the same application (master buss) exhibited some pumping and didn't let the drums "pop" through as cleanly as the API. I tried all settings of attack and release times.
 
To be fair, the PSP is a very good compressor for vocals, guitars, bass, etc. It probably was not intended to be used on the stereo mix buss, whereas the API was. So this comparison is not truly fair as I am comparing a channel compressor to a buss compressor.

In a mastering situation the compressor does not always have to have a coloured sound and I have been searching for the ultimate clean mastering compressor and the API is it.  (or one of them at least) Also the free Tokyo Dawn Labs Kotelnikov is also pretty damn good if you are after a free super clean compressor.  Xenon is pretty hard to beat as the limiter as well.
 
I can get FETpressor to sound pretty clean though but I use it in low ratio mode such as 1.5:1 and only go for about 2 to 3 dB of gain reduction.  Then it sounds pretty nice.  Keep the attack settings slow though eg 10mS to let those transients through.  I noticed once you start speeding up the attack the transients can go to mush pretty fast.
 
I set the Attack parameter first with any compressor then Release then Ratio then Threshold. ARRT! In that order.