What’s it like being a mechanic in the Army?

Yes, you will learn a lot about the working on diesel engines and military vehicle repair. You may have the opportunity to gain certifications that will transfer to outside employment.

As a commander my Mechanics loved turning wrenches. What they hated was not turning wrenches, it seemed they didn't turn wrenches as often as they would like.

It really depends on the unit and if you will be actually doing your job or not. I'm sure that you've learned by now that you can be MOSQ in an MOS but not actually be doing that MOS as your actual job.

Not a bad mos. Depending on the unit 90% of the time working is spent doing services. I am currently in a transportation unit and we have about 90 pieces of equipment. Services suck, but maintenance section is very tight group, because we are a small section. Drivers soak up the glory, we keep them on the road.
Get used to late nights.
As far as actually rebuilding anything, it is rare. Check and verify faults on 5988s, diag, get NSN to order parts and more services.
Not a bad gig, any mechanical training and skills transfer to civilian life.
And there is always temp tech and full time tech jobs possible