I had a pile of it, it was all awesome:
How dare you post strong pornography on a family forum! For SHAME! Especially a PAIR of Mono Gamas... knobs exposed and all!
Seriously though, 500 series gear is great. Is is better than rack gear? Maybe? There are a lot of 19" and 500 series pieces that have exactly equivalent circuits, minus the power supplies and IO. Some pieces are functional equivalents that have been reworked to fit the form factor and others are entirely new and different creations. That being said, ALL of them are addicting!
The basic history is that API was one of many creating modular consoles and their format (due largely in part to the 2550 op amp) just happened to be one that took off and enjoys wide spread popularity, if not quite ubiquity. There are other modular systems (like the Tonelux VRack system) that sound great, but don't enjoy the same popularity. The 500 series is a published standard that can be certified against, so you can be pretty confident that anything designed for the 500 series will work.*
My studio has a 6 space lunchbox filled with DIY modules - 2x each of Sound Skulptor MP573 (Neve flavor), CAPI VP28 (API flavor) and Hairball Lola (Jenson with a St Ives twist) and it has been a fantastic way for me to get my hands on some amazing sounding kit without paying 2-3x for their namesakes. There is a ton of really great gear out there to build if you've got the inclination. If not, you still get some cost savings from not buying a power supply and IO jacks for each unit. Win / win.
The full disclosure is that I've arguably balanced the TCO with the additional cost of tools, time and materials... but I'll be damned if it isn't an amazing feeling to plug in a sweet sounding condenser (that I built) into a preamp (that I built) and on through a compressor (that I built).
You'll pay a bit more for the chassis up front, but it will amortize out as you fill it... and you will.
* There is also an open 51X standard that can handle 500 series modules, but 51X modules are not directly backwards compatible to 500 series - I'm only including this because you'll eventually run across it if you dive deep enough into any DIY work.