I'll echo what everyone else is saying: you can't fault the quality but they're too expensive relative to comparable products and the UIs, while not unusable, aren't great. To be fair, it's not easy packing that much visual information and features into a UI without it getting messy.
 
FabFilter isn't a great comparison because the FF product line is not as broad and lacks BlueCat's analysis tools. But while FF stuff is also expensive, they've nailed the UI experience, IMO. All the FF tools I own (Pro-Q, Pro-C, Pro-L and Saturn) have become go-to plugins because of that.
 
Voxengo and meldaproductions may be better comparisons. Like BlueCat, their products are wide-ranging and of excellent quality with ugly UIs that are often not laid out in a logical way. But they're priced more attractively than BlueCat's.
 

I bought Frequency Analyst Multi when it first went on sale through Cakewalk. It works as specified with no bugs, and it does exactly what I wanted: Simultaneously display multiple audio/synthesizer tracks in the frequency domain so that I could better understand what I was hearing in my mixes, and when editing patches. I even use it to observe the dynamic frequency spectrum of external synthesizers when their output is digitized into Sonar. Frequency Analyst Pro is a quality piece of software.

I've used their Peak Meter Pro VST forever. It's really my go to for a simple meter. Also does some crazy stuff, tracking the signal and creating automation, that can be reversed to be opposite of the levels. Cool plug-ins. Yeah, user interfaces are a little on the "old skool" side. But I tried the compression several years back. It was really one of the best plug-in comps I've used. But already having so many comps, really couldn't get into another "plug-in style" compressor. The sound was killer though. But my go to in that area is always Sonalksis... for quick EQ and compression, Sonalksis is so easy to use, interfaces intelligent with the right info, and sound as good and most of the time better than anything. But I do use Blue Cat Peak Meter Pro in every session, and a lot of times pull up MultiFreq for frequency comparisons between tracks. Very cool stuff. Couldn't resist getting into their newest version of the multi-band mixer, with VST inserts on each band, too... MB7. Very cool little toy for sound design and experimentation. And Blue Cat is great with support and updates for quite a few years now.

+1 on mcdonalk.  Identical experience.