Although I've always hated listening to recordings of myself, I've been interested in trying this for a while.
Much has been said about how we don't really get a true picture of the sound that's projected out to the folks the other side of the bell. So why not stick a microphone in front of the bell and isolate ourselves from the stuff radiating backwards with a pair of headphones while we practise?
After an hour of playing around with my new toys (as described here (Audacity Trumpet Spectra)), I must say that it's quite eye-opening. Obviously, the sound is duller without all the echoes coming back off the walls etc., but it gives a much clearer picture of the detailed shape of each note: the sharpness of attack, any odd little wavers of tone and intonation here and there.
One really wyrd experience occurred while running up and down some tongued scales. They sounded okay, but then it suddenly dawned on me that I didn't know whether they were being single- or double-tongued. Without the electronic gear, I can 'hear' the difference instantly and have spent months trying to eradicate it. But through the mic, the difference just wasn't there. Had to consciously force a switch between the two a few times just to convince myself what my tongue was up to. It would be a bit irritating to discover that the differences were only in my head, and I'd been wasting good practice time working on imaginary problems!
It's a new experience, but on the face of it, it seems my ears are getting better quality raw data on which to work their magic, especially on the fine details of articulation that can get lost among the normal acoustic mishmash.
Any dangers or other caveats to consider?