I much prefer the Sennheiser mics (for live stuff that is), but I would still send it back as you should be get what you paid for. Something is nagging me to suggest that the mics could be fake, which would explain why you got sent the wrong one. Fake '58? Fake 845? Means the same thing to the seller - they've made a profit. I could be completely wrong, but its just this nagging feeling that this is what has happened. What kind of seller was it? Can we have a link to the auction?
Back to the question; I would chose an 845 over an SM58 any day, but if you can stretch to an 865 then IMO they're pretty much the perfect mic for live vocals
In my experience they really cut through a busy live mix with no EQ needed and sound great. I've only ever really used 845s on backing vox, but they sound almost as good.