To be fair: it's a big thumb up and a little thumb down. We had a really nice meal last night and will definitely return but had a big disappointment.
We ordered 5 small plates and two pastas: pork ribs, sausage with lentils, grilled pecorino w mushrooms, butter beans w puntarelle & anchovies, and fried brussels sprouts; spaghetti carbonara and rigatoni amatriciana. The small plates ranged from very good (the ribs were tasty though boring) to mind-blowing (grilled pecorino was salty & smoky from grilling, cooked just enough to make it soft without losing shape; fried brussels sprouts were as good as any fried vegetable I've had in Rome). Service was astounding: patient, gracious, and well-informed. The entire staff smiled with every interaction, no matter how small, even when we were blocking their way as they were balancing a dozen dirty plates or full glasses of wine. I've never experienced such gracious, easy-going service.
The big disappointment was the pasta sauces, especially the carbonara. Despite the praise that SPQR's carbonara has gotten on this board, I thought it worse than any I've had in Rome or made at home, and around average compared with other versions I've tried at U.S. restaurants. And this was even though the ingredient list was exactly right: pecorino, egg yolk, and guanciale (no cream, egg white, or, god forbid, peas). I found the sauce bland, lumpily textured, and lacking the usual richness that should come from rendered pork fat and egg yolk. My best guess is that there were two issues with the execution. First, according to our server, they add egg to the pasta in the saucepan over the heat before adding the cheese, so I suspect this ends up cooking the egg too much and losing the runny richness of the partially-cooked yolk. Second, the guanciale was cut very thinly and was pre-cooked slightly if at all before being tossed with the pasta. I suspect little of the fat was rendered.
I much prefer guanciale cut into chunks and fried before being tossed with the pasta so it crisps, yielding what someone on the board once poetically described as "pork puffs" -- crunchy on the outside yet airy inside. Some of the rendered fat then gets tossed with the pasta after the pasta has been mixed with raw yolk and cheese off the heat, so not to overcook the egg. I prefer carbonara to blend egg yolk, cheese, and guanciale fat seamlessly into a rich, clingy sauce, with crunchy guanciale bits for contrast.
SPQR's carbonara didn't come together like that at all. There were distinct bits of cooked egg and grated cheese throughout, which separated from the pasta strands, and floppy bits of undercooked guanciale whose flavor didn't permeate the rest of the dish. Perhaps this should continue as a thread on the general board (about finding the perfect carbonara) or the home cooking board (about making the perfect carbonara).
For the amatriciana, guanciale was cut the same way, which bothered me less since it would lose its crispiness as it simmers in the tomato sauce even if it had been pre-fried, but still.
I'll definitely return for the small plates. I would certainly try other pastas on the menu, such as the one with rabbit ragu and black kale that I'm now kicking myself for not ordering last night. But it might take a bit of time before I get over my carbonara let-down before I brave the crowds there again.