I usually fly with checked luggage, so I had not been able to take advantage of online check-in until a trip this summer on Delta. I thought it was really neat at first. It was quick/easy to go online, verify my seat assignment (which had been made many weeks before), check-in, and print my boarding pass. Once I got to the airport, it was really nice to bypass the counter and go straight to the security line. The problem was that once I got on the plane, there was someone else that had been assigned my seat. Since I was by myself, and the person in "my" seat was one of a party of three, the flight attendant found me a seat more to the rear of the plane.
Of course, this type of confusion can happen anyway. (I've seen it happen to others a number of times over the years, and well before the introduction of online check-in.) However, that's the first time it has happened to me in many years of flying, so it was a less-than-favorable first impression of the procedure.
On the flip side, I have had nothing but positive experiences in using the kiosks for check-in. On each of my last two return trips (one out of Washington National and one out of Miami), I have bypassed long lines at the normal check-in counters for very brief (Washington) or no (Miami) waits at the kiosks. And I can do that even when I check luggage. So, even if your carrier allows online check-in with checked luggage, the kiosks may be about as good a bet since you will have to stand in line anyway to check-in your baggage.
Whichever check-in method you try, good luck and have a good trip!