Carol and Lus are definitely a good choice if your looking for suits and shirts in Beijing. They have a good choice of materials and designs, and can pretty much make whatever it is you need. They aren't the cheapest tailor in town, but after using a few different ones, I've decided these guys are best. Suits always fit me nice, and feel comftable, so what more can you need?
Actually, better than the clothes, is the staff, which I guess is just as important. Really friendly people, they take time to get to know you and understand what you need, which is really important when getting clothes made, IMHO.
So yeah, after several years, and trying a few different places, I still come back to Carl and Luc, and can deffo recommend other people to give them a try.