Dear Tonio,
the annual fee is 12£ + VAT a year and from what i understood it goes as a contribution to the people putting ground stations, giving them free hardware for the installation, so honestly i think is such a small amount for such a great cause that is a plus and not a minus.
The BIIIG advantage of Rosetta is that it shows traffic with FLARM (when in range of a ground station) but, most importantly, it shows all the traffic with a transponder either mode C or S on, even if not ADSB.
This is a huge difference as today ADSB out is very limited and you tend to see only liners or very big GA aircrafts, but you miss most GA as well as ultralights.
BUT, a lot if not most traffic uses a standard trasponder, either on 7000 code, or the code given by ATC and this traffic is shown in Rosetta, unlike stratux.
of course, it can't give the exact position of the traffic as it's not transmitting the coordinated with adsb, but what it shows is the distance from you (with the code and a light, first green, then yellow, then red when very close) and very importantly, the difference in altitude compared to you (+1000, -500, +000 etc..). this information allows you to do separation as well as making it much easier to spot the traffic out of the window as you know where to look.
This is a great achievement and a huge difference in my opinion to stratux which is great when eveyone is ADSB, but we are far from that, at least in EU (USA is much better for this).
Finally, if you have ground stations, transponder transmitting in mode S without ADSB out can be seen o radar with actual position as this is calculated by 2/3 ground stations that do trigonometry with the signal of the trasponder.
For the above reasons, really can't wait to have the Rosetta traffic in FLY IS FUN as it is the best combo out there!