I was using Density MkIII by Variety of Sound [just for its smooth compressor] and was looking to see if I could improve on that without spending too much money. I own a number of ToneBoosters' other plugins and I'm really impressed with them. In the end it came down to a choice between this and Klanghelm's DC8C which looks great and gets good reviews.
In the end I opted for the ToneBoosters BusCompressor for two reasons - it does everything I need without superfluous complexity, and Klanghelm don't offer a free trial, whereas ToneBoosters offer full versions of their plugins to try before you buy. I'll probably buy DC8C on another occasion but for now all I needed was a decent transparent compressor without too much complexity. TB BusCompressor fits the bill perfectly - it has all the normal options you expect to see. In addition it has a few 'expert' setting for transient release, filtering the level detector, upward compression and so on. It looks and sounds better than any other compressor in its price range (including all the free ones I tried). I'm surprised that it's not recommended more often.
Just like all the other TB plugins it sounds neither good nor bad - it just sounds 'right'. Exactly as it should. I like the ability to set the transient compression percentage - really helped me out.
Being really critical I'll mention two things. First TB plugins don't have an undo feature - I'd like to see this. Second, TB don't cover all their plugins in the downloadable manual - this being one of them. I'd like some documentation to cover the specifics of the expert settings. But neither of those is enough to detract from an outstanding product, so overall I'll give it 10 out of 10. If you want a coloured 'effect' compressor then you'll need to look elsewhere - but everyone needs a great sounding transparent compressor and this one should be very high on your list.
Update 19th Dec 2014.
I've just upgraded to the latest version of this plugin. The GUI's had a nice overhaul, but there are also a couple of very interesting functional changes. The release curve now has additional fine tuning parameters, and the hold time is now measured in cycles (rather than milliseconds) which helps eliminate intermodulation distortion. Most crucially, for me, is the really clever Noise control - the plugin analyses the signal not only in frequency but also by harmonic vs non-harmonic (noise) components. You can then use the Noise setting to weight the amount of compression given to each component. On vocals this 'Noise compression' acts more on the sibilant components with makes a noticeable difference to the amount of de-essing needed. In some cases it eliminates the need for de-essing altogether. I don't know what else they've improved but it sounds even better all round. This plugin just moved up to the next level. If it only had a sidechain input it might just be my desert island compressor...