I've only run two Flashpoints so far - Esseles and Hammer Station - but I'm having fun with them. Esseles lures you into a false sense of security, I think. It's not that difficult, as it's probably designed to be a sort of tutorial on Flashpoints. Hammer Station, however, proved to be quite the challenge and my friends and I enjoyed it. First two of us with companions tried to tackle it and wiped a few times. Then we brought in a third person and STILL wiped a few times on the last boss. We got him eventually though. It's been years since I've actually had to learn an instance and its boss fights and it's pretty rewarding when you figure it out for yourself rather than looking up videos online.