I am biased since I work in a shop that sells Felt, and I own one. Compared to (some) other brands Felt seems to offer a bit more for the price than it's direct competitor. They are a very popular line for us. They did cut some corners on certain parts to keep the price down. The wheels (CXP22) are a bit heavy but with a good truing they seem to hold up well, and that is a plus. The crank is an FSA, and stands to be improved upon. Great to start with, and easy to change when the first major bearing overhaul is needed. The Microshift brifters are not bad, and coupled with the 105 RD works nicely. The only issue that I see with the drivetrain is that the rear cassette is an 11-25, and even when coupled with a compact crank doesn't offer much for a bailout gear for the unseasoned cyclists it commonly sells to. The seat is a torture device, and should be changed unless you happen to have very narrow sit bones.
I really cannot comment on the other brands, as I am not nearly as familar with them. I have owned both of the other brands in the past, and was quite happy with the Giant I owned. The Cannondale that everyone seems to talk about is the CAAD series, IDK about the Synapse.