It's a decent start, but it's actually not as great as it looks. Once you improve all the proper squares, you've only got a +3 food surplus, which is only enough for 1 1/2 of the mines. And you've only got 2 river squares that'll give extra food via improvement. So you're city is going to be pretty food-starved early on.
At size 7, for example, even if you've farmed the two grassland/river tiles, you're still going to only have +1 food, and you won't be able to work all the mines (or you'll be losing food).
Once Biology hits, that city will be a true powerhouse. And it's still a better than average start, that's for sure.
Bh