When I have them, they're way more intense than when I ejaculate. I don't have them often, but when I do, I usually have a couple of them. They feel great during the process, but there are two things I hate about them.
First, a LOT of guys just don't get that they can actually happen, so I've been accused of faking it. I guess if they don't see jizz, they don't think an orgasm actually happened. Mostly I have them when I'm with someone, though I have had a few while jerking off. One night I had 4 of them in about a 2 hour period and then went on to shoot a load 2 more times. The guy seemed pretty impressed with himself. LOL
Secondly, when the session is finally over with I end up with this KILLER headache and feel really weak. During the orgasms it's totally worth it. For the hour following, though, I kind of question that.