I threw my bid in the hat for upgrading on the first thing I could get, and landed the Ultra.
I've been on the fleet coming up on two years (came out of the XL - apu aircraft as an FO).
I'm very happy on the Ultra. As the others have said, the workload is very manageable. The 400 guys apparently sit around alot (not a good thing, necessarily, especially when doing Hot Spare).
The Ultra breaks enough that you can sit around plenty.
I usually get one or two days a tour sitting around with broken plane, meaning hotel time.
Compared to the XL, the lifestyle on the road is much, much nicer. Last tour, the earliest show I think we had was a 8 am on the first day. Again, there are fewer Overtime opportunities as well, but as a new hire, this is not really as large as a gain (meaning, your pay rate for OT isn't high enough to warrant the ballbu**ng schedule).
The 400's smell better inside, and have TCAS.
Come on over! I'm very happy I ended up flying the 560 "Classic" as opposed to the 400. Just one opinion.