I flew a few hundred hours in a Stemme S10V (no turbo, Limbach engine). Based at my club in Varese, I was able to reach the Atlas mountains in Morocco in just three flights.
When using the engine, the running costs are quite high, because of the propeller's limited TBO which significantly adds to the engine's overhauls. I've been told that TBO has been increased so its impact is a bit less.

The Limbach engine performed adequately but has single ignition. It is best suited to long, hard runways and not-too-high elevations. No problems taking off in Ouarzazate with 40°C (hot) at 1.300m MSL in June, two on board and slightly over max mass. The runway is 3 km long, however.

It's a side-by-side cockpit and that implies a few remarks. Of course it's more "social", but not plenty of room for two large male adults. Seating position is very comfortable, after I added cushions to support my thigs and knees. Visibility towards the side opposite to your seat is significantly less than in a tandem cockpit.
Also, Center of Gravity moves forward quite a lot with two people on board; if it's setup to fly safely while solo, it will be heavier on controls and require too much trim and stick when flown double. If setup for double, it will be impossible to fly solo (exceeding rearmost CoG). I enjoeyd it the most when flying solo.
Handling gets lazy with positive flap. I used Zero until I was well established in a decent thermal, and then added flap to reduce speed and circling diameter. Any corrections are much easier with zero flaps. Use landing only on final, after having found the correct alignment.

An Italian guy used it with excellent results for a few years in the Open Class championship. He stopped this activity when handicap factors were abolished for italian national championships in any class except Club and TwoSeater.

I miss the Stemme.

Aldo Cernezzi
www.voloavela.it