We always warmed it in a bowl of boiled water from the kettle. I'm not sure how fast the bottle warmer is (I think it's just a little electric kettle of sorts, right?), but it didn't take too long (5 minutes total incl time to boil the kettle). If you really need it quickly, you could always just use ready-made cartons at night, which would be room temp, so not 'warm' but not cold either. We would do this on occasion when we were traveling somewhere without easy access to a fridge or kettle during the night. It is more expensive, but I depends whether it's worth it for the convenience.
My daughter is 14 months now, so is on cow's milk not formula, so I warm her milk in the microwave which I'm comfortable with. But I don't know that I would have done it with formula, as microwaves can degrade some nutrients (more of an issue when your baby needs to get their nutrients from milk rather than food or vitamin supplements) and I didn't like the idea of opening and closing the cap a lot (making it less sterile - plus our microwave is gross anyway). That's just a personal choice though.