To those using the Vortex Spitfire AR 1x prism...

What is the eye relief like?  How forgiving is the eye box?

Can you mount a Magpul MUBS behind the optic or is the scope then too far forward to be usable.

I have a terrible astigmatism and am thinking about giving one of these a go, however, I want to make sure it can be used how I want to use it.

I have both the 3x and 1x and the 3x does have eye relief that is a factor. The 1x not so much. I had it mounted further forward and it was fine. It does have eye relief though, unlike a regular red dot. But to my eyes it is not a problem at all.
That said, I have read that they don't work that well with BUIS. I don't know because I have never tried it. Since this optic has a visible reticle even when it is off there is really no need for BUIS. I don't install any on any of my rifles that have a prismatic scope because it is extra weight, might not work that well and really isn't needed.

So how far forward to you have it mounted?  Far enough for a back up sight?  I know you can't deploy BUS with a prism scope, but if there was a catastrophic failure, the scope could be removed.  

I put back up sights on all my rifles out of habit.

I have mine set at the very front of the receiver. The flip up BUIS works just fine and the 1X can be set to co witness OTB. No eye relief problem at all with this arrangement for me..

Mine is set up further back but I keep my face further back than others too. On mine I could not fit a BUIS as there is no room left currently. Just personal preference. But as the other poster said, I too have it more forward with no problems.
But remember, one of the selling features of this kind of scope is that if it goes tango uniform with dead battery or broken switch etc ... you can still see and use the reticle. Unless you are parachuting into Iraq, this should be fine. I did have a dead battery at the range once and forgot to bring a spare (never happen again) but I was able to shoot and have fun.

However to each his own. If you are a belt and suspenders kind of guy then you should be OK with putting a BUIS behind it. Especially if you are a close up shooter that does not extend the stock much and keeps your face real close to the charging handle. My buddy is like that. He shoots with the stock completely collapsed to its shortest setting ... likes it real up close and personal. For him he could have this thing way forward.

Yeah, I usually keep my stock at the second setting but this rifle I used a Magpul Fixed Carbine stock which sits at that setting anyway.  I just feel better knowing that I can deploy a back up.  I am not an operator operating operationally, however, I like to build my rifles like I am.  I usually use an Aimpoint PRO or a Meprolight RDS and they work great, but want to try something different on this rifle due to my astigmatism.  I have heard good things about this scope from others with astigmatism.  I also have a Sparc AR, but it gives me tunnel vision.  I prefer the full sized red dots and the FOV with the Meprolight RDS is unbeatable.  It would just be nice for once to actually see a reticle as it is supposed to be seen.

Its good to hear that I can mount it farther forward with no issues.  I think that seals the deal for me and I am going to try one out.

I shoot exactly like your buddy...stock forward...nose to the charging handle so...it works great for me forward. To each his own there. Mine has the A2 sight post and the flip up was mounted well before I ever considered an optic for this weapon's general use. It is truly a redundancy but since it was already set up with BUIS...I left it as is. It can be co witnessed (as mine is) but you are completely right about the beauty of tis little Vortex having an etched reticle which actually makes any other sighting system useless/redundant unless it is totally removed.

There is no real 'eye relief' to speak of.  However, it needs to be mounted closer than a typical red dot to get the full view depending on your normal cheek weld.  Dump the BUIS, it is not needed with prism type optic, the etched reticle is either 'black' in unlit condition or red/green in lit condition.

BTW, etched reticle with adjustable eye piece solved my astigmatism problem and I've been competing in 3-gun with the Leupold Prismatic in the past 4 years with it. My Vortex is on the training gun.  Neither one has BUIS.