On a technical level, it's impressive that Microsoft has been able to get Xbox 360 titles working on the Xbox One at all — emulating modern console architecture is a hugely challenging task. But now the feature's available, how well does it actually work? As Eurogamer's Digital Foundry reports in an extensive investigation that's well worth checking out, the results are mixed.
On the positive side, the Xbox One implements vertical sync across the board, meaning that Xbox 360 games run without any screen tearing at all. 1080p upscaling quality sees a "slight improvement," while games that require disc and hard drive access often perform faster — this makes sense, since Xbox 360 games need to be completely installed to the Xbox One hard drive even if you still have the disc. The original Gears of War is cited as a title that benefits from faster texture streaming, though you might want to check out the Xbox One-specific Ultimate Edition if you're going in fresh. Hydro Thunder and Condemned are also reported to run better on the Xbox One.
Halo: Reach and Gears of War: Judgement are poor performers on Xbox One
Other titles don't work quite so well. Gears of War: Judgement and Halo: Reach are both described as "nearly unplayable," with major performance issues causing the frame rate to plummet; the latter game's issues are particularly disappointing, since it's the only Halo shooter without a dedicated Xbox One version. Mass Effect sees improvements in load times and exploration sequences, but combat is "a mess." Gears of War 2 and 3 are considered playable, meanwhile, but you'll see some slowdown.
Read Digital Foundry's full report for more detail. Overall, Microsoft has done good work in bringing Xbox 360 games to the Xbox One, but it sounds like the company could do a better job of green-lighting certain releases onto the compatibility list — and if you still have your 360 hooked up, the older system might give you better results. Still, it's worth noting that Microsoft's emulation could well improve over time, and in fact already has done so; some games in the Digital Foundry report turned in better performance than when the feature first launched. Here's hoping the same happens with Halo: Reach.