I had been banking with Corporation Bank since 2000. I must say I always found their service to be quite good. They don’t over promise or under deliver. Their internet banking user interface is not very elegant or very look-attractive. But on a different look, they have every feature on the internet banking which people like ICICI or HDFC boast off.
I have personally interacted with corp bank staff of at least 6 branches across 3 different states (Karnataka, Andra Pradesh & Tamil Nadu) and all places I found them to be very helpful and very cordial people.
If one ignores the size of the bank, but look for a pleasant banking experience, I would more than recommend Corp Bank.