My mom and me stayed at Sergeys apartment for three nights. Loved it! Would definitely recommend it.
So first things first: Sergey is a great host. If you have any questions, he'll answer it. If something is wrong during your stay, he'll fix it - or at least he'll make an effort. He didn't have time to meet with us personally at the apartment, so he sent a friend, who also gave us some great advice on where to go in Riga.
The apartment itself looks exactly as in the pictures. It's clean, nice, and well-equipped. Everything you could possibly need, is there (nice detail: we even saw an umbrella lying around). The street is quiet, but very central within Riga's old town (so if you're a light sleeper, I would advice on bringing some earplugs, just in case).
The only downside we could find, was that the shower never really gets hot. We had some problems the first day, but Sergey fixed it immediately. However, the water just remains lukewarm, so I think that it's supposed to be that way. Still good enough for a quick shower, though, so no big deal, just a small inconvenience.