I love the metronome as a learning/awareness tool during personal practice or group rehearsal time.

My a cappella quartet has had an experience similar to yours, GSS, feeling startled by the difference between what felt like a steady tempo and what WAS a steady tempo. We were speeding up and slowing down as we rehearsed "Orange Coloured Sky," which we intend to sing with a steady backbeat.

The first couple of times we used the metronome with that song, someone would reach over, pick up the thing and peer at the dial, certain that it had moved.

I've used them with piano practice as a kid, from time to time with my own guitar, banjo & cello efforts over the years, such as they were, trying to make even fingerpicking. Also I've used them in vocal ensembles and handbell choirs. That experience has taught me there are several reasons for inadvertently changing the tempo, the least likely being "musical feel." Most often people speed up where they are more familiar, slow down when they have to think more (often unaware of it), speed up when there are many repeated notes, speeding up when playing/singing loudly, slowing down when singing/playing softly. Those are the biggies that come to mind right now, there are probably more.

If you hate metronomes, you can still improve your playing by being aware of why it happens, so you can make musical choices. I find the more I make informed choices about when to speed up or not, the happier I am with the magic that happens at performance time. I'm saying sometimes it's good to resist the natural tendency to rush a passage, it can create a lot of good musical tension, really draw in your audience (they'll probably feel that urge to rush as well), then we choose when to let them go instead of losing that opportunity entirely by never learning it was there.

I hope that makes sense to someone besides me.