Hi!
I have been running and servicing MVVS engines for more than 10 years now and what I have noticed over the years is that MVVS engines are very durable.
In the last two years MVVS have improved their quality drastically with their new CNC machines which has rendered in better carbs and more powerful engines.
Some of the MVVS line of engines (6,5cc) are perhaps not as powerful as other engines (read webra .40 GT). The 6,5cc GFS/R engine for instance is not as powerful (at least with the standard and super silent muffler) compared to a Webra GT .40 but with the new updated standard carb (7mm, 8mm or 9mm intake) it certainly performs very good having now (really) instant throttle response and super reliable idle. But it lacks in rpm compared with the Webra .40 GT witch is the most powerful "standard" engine on the market. -We use this engine in Q-500 pylon racing in Sweden.
The Webra .40 GT turns a 10x6 Graupner prop at 14000 rpm or more on the ground using 80/20 fuel (FAI) and the latest MVVS 6,5cc do 13600-13800rpm with the same prop.
Small difference maybe but noticeable in flight.
What is good is that the MVVS carbs have been improved and is now one of the best on the market rivaling OS.
This was not the case with the earlier carbs with their blunt idle needle..these were rather inexact to say the least.

I think the super silent muffler is good, it's very quiet even quieter than a fourstroke if you put on a the right prop that is (11x6 APC on a MVVS 6,5cc GFS/R).
This silencer is not riveted together ...it's bolted together...which is much better.
There is however some minor faults with the smaller MVVS engines (3,5-8,0cc) and that is the bolts used to secure the silencers! These bolts are to small in size (M 2,5 and M3 )and they are also too short.
I hope MVVS will change them in the future.

Regards!
Jan Karlsson
http://hem.passagen.se/airracing