I am about to undertake my seventh ultra, but you never forget the first.
The feeling of fulfilment on finishing your first is proportional to the challenge involved. You know that you have done something that relatively few runners in the U.K have done. You just don't get that sense of satisfaction from running a half marathon.
Also you will never find a friendlier more inclusive group of people to run with. The nature of ultras and the pace they are run at makes them much more social events.
Be forewarned however that they are very addictive. Once you have done one you will almost certainly want to do another!
Thought I'd post on this as it popped up as it were and quote the second reply from Ben 
He's right, I haven't forgotten my first ultra, but seeing as I've only done one it's not surprising
I did a 38 mile off road route as part of the Jedburgh Half Marathon weekend last year, mainly because I live in Jedburgh and it was the first time it had been run.
It was painful and exhilarating at the same time, words sometimes don't come close to the emotion I felt during and after the race.
The other runners were great and sense of comadarie was brilliant.