Hunter:
As to whether you keep your tube amp once you have an AFX, no one can ultimately make that call except you. Some guys say they still hear nuances in their tube amps tht they prefer, many more who post opinions say they don't use their tube gear nearly so much, even if they hang onto it.
I'm not one of those who want to convince others of what their reaction will be - you have to try it for yourself. I will say, though, that there are a couple of huge factors:
1. Most AFX owners agree that, at the very least, it comes scary close not only to the sound, tone and feel of a good amp, but many flavors of good amp. So versatility is great. And most users feel that even if it doesn't precisely duplicate their fav amp, it's as good, just slightly different. (Two samples of the same actual amp rarely sound identical, as well.)
2. Convenience: with the Axe FX, a full gigging rig can consist of a small rack case (a 3U rack case can hold the AFX and a 1U power contitioner, e.g.), a powered monitor, and a MIDI floorboard. That's it. And you can simultaneously have great stage sound, direct feed to FOH, and have it all sound the same. No tweaking two different patch variations for stage and FOH. And if you only use in-ear's, you can do without the monitor.
3. Low volume use: all tube amps have to be cranked to sound their best. Big limitation, whether you're a home player, or just always fighting with the FOH guy about your stage volume. With the AFX, you can have the sound of a cranked rig at any volume, even bedroom volumes. (Keep in mind there's no subsitute for a certain amount of volume to flap your pants, but that's a different consideration.)
Re: FRFR (full-range, full response) vs. traditional guitar cab amp, my experience is that you will have a very difficult time getting the most predictable results without going FRFR. If all you want is a few of "your" tones, and you tweak those carefully to your rig, you can get what you want. But you'll be more limited in wider varieties of sounds than the FRFR guys. Audio is my background, I have access to analyzing equipment, and I've spent a good long while considering traditional vs. FRFR with the AFX, and FRFR is better for me.
I was one of the people who picked up a QSC HPR122i powered monitor early on, for use with the Axe-FX, it's one of the most neutral powered monitors out there, at any price. (Most PA monitors are not really that neutral, FWIW, I rented a different model for a few days and wasn't at all happy - took it back early.)
There is a paradigm shift taking place here. Experienced players who are used to traditional rigs are finding it difficult to envision not having tube amps and guitar cabs. That tends to change once they dig into the Axe-FX, discover what it can do, and then start working on how to maximize it's capabilities.
But you have to make up your own mind, for your style of use.
Ultra vs. Standard: I have a Standard, and I wouldn't have much interest in the more exotic effects that only come with the Ultra, but the Ultra does have more memory resources, if you feel you might be setting up patches with dual amps and/or very long effect chains. (Did I mention that the effects are gorgeous sounding in the Axe-FX?)
Doing it again, I might get an Ultra if only to ensure it doesn't run out of resources for future firmware upgrades, but this applies more to effects than amp sims.
Someone mentioned that a lot of the clips are U2 sounds. Not true at all - there are a ton of clips on all styles. On the AFX forum, there is a fellow named Jochen from Germany, who posted a bluesy clip using the Trainwreck sim, using nothing but the guitar's volume control for control of drive and dynamics, and it sounds incredible. Lots of samples available, if you dig thru the forum.
In the end, all that will count is your own opinion.
Brian