I have, as an original owner since 1976, Roland System 100 (basic unit 101, expander 102 and sequencer 104) and just studying how to use that together with Behringer System 55. After some first tests this works well. Similarly, the very small Pittsburgh System 10.1 (Eurorack module) is working well with this System 55. Some thinking was required with the trigger signals, but once I figured out S-trig versus V-trig differences then it is quite easy to use different systems. The sound of System 55 is very good. Luckily I bought a lot of additional patch cables as there was none with the System 55. I have the feeling that some more mixers in System 55 would have been a good addition. But as there are three wide blank plates (together 140 HP plus 2 of 2 HP blank plates) it is easy to add what modules I want/need additionally after experimenting more. The MIDI-module is an excellent addition to this collection of miniature Eurorack modules based on an historic synthesizer. The low price of Behringer System 55 was one of the reasons to buy this unit. I have been thinking a long time to have a big Eurorack system, but the price always got too high when I was figuring out some system of modules I would like to have. Here we have a good combination for many purposes (Berlin school style, etc), but that is not always the best unit to start an Eurorack system if one does not have much hands on experience with sound synthesis. Personally, I don't regret getting Behringer System 55.
As a negative point I have to say that it is somewhat tedious to work with this system as my huge hands are like two bunches of sausages. Obviously, the same problem would be with almost any Eurorack system. Still, I have been using so long time VSTs on my computer so that I feel that the use of mouse is not so comfortable way to work. Hands on physical knobs and switches makes the creation of sounds (and hopefully some music) feeling more realistic.