From what I've seen the Magma unit looks good and it is useful to have three slots but the trade comes as it is quite a bit bigger than Sonnet Echo Express and a lot bigger than M-Logic RR. The Sonnet unit I have and so far am really happy with, it is larger than M-Logic but when you open it up I noticed it has a pretty beefy power supply for the PCIe cards that demand a lot more juice and has three big fans in it. The M-Logic is really nice being so small and I can imagine it being a great laptop bag solution. However I've noticed the other two units are bigger probably due to the power supply and fans, not to mention you can put any card you want in them.
They all have they're pros and cons, Magma is great for desktop setups, Sonnet is great for desktop and laptop setups and like the Magma can take any card and the M-Logic looks like it will be a good RED Rocket laptop bag unit.
That being said, the Sonnet one is the only one actually shipping and the only one I've received and tested and I like the performance I've got so far.