The problem with Bun Saigon isn't really with their food, but rather that there are multiple equally good or better Vietnamese places within a 2 minute walk. Everything I've had here I would describe as entirely fine. I'm not sure how I feel about the thicker noodles they use in their bún. Some of their less traditional dishes are almost good. The atmosphere is decent and the staff are nice.
In terms of food, though, I'd rather walk up the street to Pho Hung (unless you're particularly bothered by uncaring to downright bad service and waitstaff). If all I want is pho, I might hop across the street to Pho Ai My instead -- their broth is great. If I want the best Vietnamese nearby, I'm walking a bit further up the street to Anh Dao.
Bottom line: you won't regret eating here, but you can probably do better.