For its price point at the time of purchase (£4 than the XR18), the MR18 was a worthwhile upgrade over the Behringer XR18. For this review I'm only going to explain why I think the MR18 is better than the XR18.
The pre-amps sound fantastic. The first microphone I tested them with was an AKG C414 on vocals and acoustic guitar and I was blown away. They are in my opinion better than the pre-amps on the X/XR series (which are already very good) and they are considerably better on some older digital mixers (Yamaha 01v96i in particular).
The genuine neutrik locking xlr connectors are fantastic and mean I no longer worry about cables getting pulled out and the 10 year warranty is also fantastic and gives me great confidence.
The wifi is sadly as poor as on the XR18 and you will need a wireless router with you to ensure a strong, reliable and secure connection. The MR18 also lacks hi-z inputs (the XR18 has these) but this is because the Midas Pro Preamp design lacks the circuitry for hi-z, but I prefer to use a DI anyway - hi-z was just a nice 'just in case',