The Pho is outstanding, the spring rolls are fresh, delicious, and plentiful, and the atmosphere is exactly what you look for and expect when going to a Vietnamese restaurant. I personally love the demeanor of the owners/cooks/employees. They have a very direct and no-frills approach to their business, but they always deliver an excellent meal. I first visited two weeks ago, and I have already been back three times since. For anyone who is less adventurous with food, their Chinese dish offerings are surprisingly delicious. My roommate is not a fan of Pho, but has gone with me three times to Khai Hoan because he loves their Kung Pao Beef. The Pho Tai Nam is my personal favorite.
One note: Make sure you take cash if you plan on spending less than $10 (which is easily possible, given that any Pho dish is less than $8). There is a restaurant policy that anything charged must be greater than $10. However, I highly suggest ordering the spring rolls along with any dish. You will spend the allotted minimum and come away more than satisfied.