I did this with a bock. I collected half the run-off, which took a bit more than two hours. I put it into a 750ml bottle with a Grolsch-style flip cap, then left it at room temp for three weeks next to the bottled bock that was non-freeze concentrated.
The regular bock was about 7%, so I figure the eisbock is ~14%, give or take. It doesn't taste too hot or alcohol-y. I have tried freeze concentration with wine and that is definitely a problem... but with the eisbock it wasn't an issue.
The mouthfeel was almost viscous... definitely different than the native bock. The mouthfeel reminded me of the ice wines my wife and I tried on our honeymoon in Niagara-on-the-Lake, but it wasn't *quite* as good. Still, a lot better than the un-concentrated beer (not that there was anything wrong with the un-concentrated beer... it's just a normal bock).
The mouthfeel was the
only thing that was better. The freeze concentration intensified the hop flavor, so it ended up being too bitter despite the extra sugar that was added before freeze concentration. Why did I add extra sugar? To see if it would carbonate... it did not. Opening it did let out a little CO2 hiss, but that was all. Maybe if I had left it longer it would have carbonated properly, but freezing killed off enough of the yeast that it might have taken months to bottle carb, who knows?
It tasted not very good... too bitter. I'm a bit bummed that I wasted 1.5 L (about 50 oz, or 4+ beer bottle's worth) on this experiment. I did learn a lot, but I think rule #1 is that
the beer must be malty, and
hops are not an eisbock's friend. Also, if you want it to carb in a bottle you need to add back in some live yeast (maybe a couple ounces of the unfrozen stuff) or wait probably about 7 times as long (21 weeks?).
That's how it worked out for me, YMMV.