I play in a traditional Brass band, and I've always wanted to test out a rotary valved instrument, hence I took a punt on the Thomann flugel. I normally play a Bach Strad flugel with a Denis Wick 4F mouthpiece. In comparison to this, for a sub £400 instrument, there are a surprising number of benefits to this flugelhorn.
Firstly, the rotary valves are excellent, quick and responsive. In rehearsals some technical passages which were slightly problematic on the Bach were simple on the rotary valved instrument. Also as far as tuning and intonation goes, I've had no major problems, nothing more than playing on the Bach. In rehearsals I used a Denis Wick 4FL mouthpiece - ok this piece is not fit for a german shank (didn't want to invest in other mouthpieces just yet), but the sound and response of the instrument were still great.
Further, I found that the mouthpiece supplied with the flugel is servicable. I actually prefer the slightly brighter sound from the medium deep cup to playing with the Denis Wick, which I used in rehearsals to blend in more with the band. I've since used the mouthpiece that came with instrument in rehearsals and performances and had no major issues with it. An interesting feature of the instrument is the water key, which can be useful for tuning some high notes F, Ab (alternate fingering), Bb, D above the stave.
Cons - Holding the instrument in a horizontal position leads to quick build up of water due to the narrower bore, and it then can't be removed quickly, which is not good for longer pieces or solo playing. Hold it tilted to the side slightly to correct this, otherwise you'll spend a lot of time during short resting periods frantically removing water.
Finally, I don't know how our German and Austrian friends play this type of horn without destroying their hands! After a few rehearsals and performances I've found more comfortable ways to hold it, and find myself adapting. However, don't underestimate this issue if buying this type of horn. Piston valved instruments are definitely more comfortable.
As far as the instrument goes, I've no real complaints. For the price, the quality is very high. Also the service from Thomann is first class, and I would definitely use again.