Originally, I just got SpinAudio Fx-Designer, but since SpinAudio discontinued selling it separately, they offered a free upgrade to their essentials bundle which includes besides FX-Designer their SpinEQ, SpinDelay and, yes, Roomverb M1. Well, very generous SpinAudio! The whole suite is superb and a very good value for money.
But back to the Roomverb M1. The M1 is the predecessor of the Roomverb M2. Actually, as SpinAudio claims and what I also believe by testing side by side: the M1 has basically the same sound engine but just lacks some advanced features (like reverse reverb etc). For me that is not a negative thing since the M2 has too many of them. The only thing I miss is being able to adjust the out level (you can adjust the in level though), but that's not a major thing.
The Sound: it is almost all I need from an algorithmic reverb. There are many controls, and the user interface makes it really easy to edit. The really weak part are the presets. These do not show how nice and smooth the Roomverb M1 can sound. So I did some which imo show this better. You can download them from:

http://www.power-xs.net/download/roomverbpresets.zip

The documentation is nice and I had no stability issues. Customer support was therefore not needed so far (but in other regards SpinAudio was not very responsive lately). The value for money is incredible. The Roomverb M1 alone is worth the money they are charging for the bundle, and FX-Designer (you can use this as a multitap delay, but it is a very flexible unique tool) and the SpinDelay (true stereo delay with many features) make a valuable addition to any fx folder. SpinEQ is ok, but there are better ones out there.

To sum it up: the Roomverb M1 (resp. the Spinaudio essentials bundle) is a good product for anyone who needs a good algorithmnic reverb, but does not want to spend the money which other companies are charging for a similar sound quality. But be aware: the M1 is quite cpu intense. So don't through away more cpu friendly freeware gems like the kjaerhus clasic reverb or glaceverb (which has a very special sound).