First time to St Petersburg
Highly recommend the blini shop Samovar for lunch. It's a neighborhood cafe, much better than the other blini we had there, and according to my native Russian companion as good as homemade. Ulitsa Gorokhovaya 27, near Kazanskaya. A few blocks off Nevsky Prospekt and well worth the trip. I don't think they do English; but you can just point at the fillings on the counter if you can't translate off the Russian menu.
Kavkaz Cafe/Restaurant/Bar has decent Georgian, if a little touristy. Ulitsa Karavannaya 18. They have an English menu and take credit cards.
Dvorianskoe Gnezdo is a dotingly fancy french-russian restaurant. A tasty conservative high-end meal; for $200pp you can get similar food anywhere in the world, though DG does offer the novelty of having bear meat or beluga in some of their dishes.
So we are currently in StP and leave tomorrow. So far we've had mixed luck with restaurants. We've been going out of a guide book from 2 years ago that seems to be out of date a bit (like with tipping, restaurant quality, expensiveness, etc.). On top of that, a lot of the restaurants we were most excited about are closed- Onegin (hip russian/european), Lucky Shot (meat), and Bagration (georgian).
We really wanted to try some georgian, so the only other georgian place recommended by our book was kavkatz bar (lower nevsky prospekt). The inside was kindof tacky with fake decorations. The food was quite good- the marinated carrots wrapped in eggplant were awesome, as were the giant puff pastries filled with onions and mutton with garlicky red sauce (something like tsubeki). The khalichki was so-so. The food was very expensive, compared to other places. Don't bother with alcohol- they don't even have georgian wine and the tiny glass of chilean wine ended up costing almost 20$. Mildly recommended: Some very good dishes to try, but not a great place to chill out.
Another place we went to was serbian/russian Black Cat / White Cat (a few blocks north of lower nevsky). This was a pretty chic place, and their barbequed meats were awesome- try the meat assortment if you can. The staff was also very nice. Definitely recommended.
For lunch, we went to a place on lower nevsky named in russian for teaspoon. They had all types of blini and a bunch of cold salads, probably more but we were hungry, and it hit the spot.
Also, unexpectedly went to a place on nevsky called Dve Palochki which was a sushi place. Maybe it was because we had seen pictures of sushi at every wannabe trendy restaurant already in st petersburg, but their sushi wasn't bad, and they had a pretty extensive japanese menu too. Every time we passed it the place was packed. The interior of the place is very cool, though the bathrooms with two toilets facing each other were a little strange. I think it was the only place in StB where I would eat the sushi.
We also went to the literature cafe. This place was very historic, but that was about it. The service was awful and the food seemed like it was microwaved. Maybe a good place to sit down, get a drink and order one of pushkin's favorite dishes. Warning- the english translations on the menu are horrible. The only thing i'd recommend is the baked mushroom- mostly because it is served nicely.
Last place we ate on nevsky prospekt is Ili. It's on a corner about 2 blocks past Gostiniy Dvor, and is more a trendy cafe with a cool interior, free wifi, a nice drink menu, and some decent food. The mushroom soup was very rich and good.
The only other place where we ate out was a random hole in the wall bar/restaurant where we got soup and beer. I think for the price this was the best place we ate during our stay. Our meal with beer was all of $20, and they served hot rolls and some kind of homemade bread chips. We also stopped in random bistros for quick snacks and they were all pretty cheap and good as well. We may have gotten lucky, but this may be the way to go when you want some honest russian food.
Also, in nearly every place we ate at, the russian beers were decent and i was told the soviet semi-sweet champagne was good. Both were pretty cheap around 100r per glass.
Hope this helps. Good luck.