It feels and sounds like an amp. Full stop. I didn't notice anything different, other than not having the sound coming out of the 4x12 on my back.
Compared to my amps? The closest is my Dirty Shirley Mini, for obvious reasons. The Marshalls are different, but they're also different from each other, that's the point of having different amps. :)
But the IR-D sounds like a legit tube rig because well, for the most part, it is one.
I've had a bunch of modelers, a Kemper etc and I didn't like any of them, I had all but given up on DI rigs. A lot of people like modeling, so I won't get into it, but it's just not my thing.
But I don't feel like I'm compromising anything as far as the live sound goes by using the IR-D.