I don't believe I've ever given a single item I've reviewed five stars, but the X1R certainly deserves all the kudos it gets. It's very warm and "vintage" sounding (as you'd expect from a ribbon mic), but it certainly doesn't sound muffled as I've had other ribbon mics come across in a mix. I've used it on both vocals (voice overs) and guitars and have been more than pleased with the results.
Mixing it with an SM57 (using the GuitaRF isolation shield) is super simple, but, frankly, I didn't feel the need to use both mics as I didn't feel I needed the fizziness I associate with the SM57. It stands on its own in that situation and is superior when you run it without the shield in a fairly live room. That hits that Bo Diddley vibe just right!