I've got both the Bosch 12v driver and impact and the equivalent Milwaukees. I also have the Milwaukee Hacksall.
I've had the Milwaukees for a couple years, but a few months ago Bosch came out with their newer, faster, more powerful models, and I saw a set on Black Friday with a ridiculously low price, so I bought them to use at my workshop.
The new Bosch units are way more powerful, and much faster running, but after using them for a couple months - not extensively, but enough to get a feel - I still like the Milwaukees better.
For some reason the Bosch units just don't seem to be made as well. First, there is a delay when you pull the trigger. It's slight, but noticeable. Second, the forward/reverse switch seems to have a lot of play in it - often I have to mess with it to get it to work right. Third, the quicklock chuck on the Bosch's aren't as good, and this may be a problem with mine only - but a couple times when I left the driver out in the cold garage, it wouldn't work at all. This happened this weekend. I picked up the milwaukee and it worked fine. The Bosch driver doesn't have the battery gauge - which curiously is included on the Impact and is on all the Milwaukee units.
Also, it may just be me, but it seems the Milwaukees have better battery life - of course that may be due to the fact that the Bosch's do have more power and speed.
I'm looking forward to the next gen Milwaukees, that are surely going to be released this year.
As far as the hacksall - I have a mixed review. It does accomplish light duty cutting way faster than you can do it by hand, and it is way more compact than a sawsall. I've used it to cut copper and PVC tubing and for a multitude of other small jobs. It's not very powerful though. It vibrates like crazy and the battery life is pretty bad. On my last use, mine developed the habit of shutting off after about 5-10 seconds of cutting - I recharged the battery and it stopped, then later it started doing it again - must be an electronics problem. I was really looking forward to it when I bought it, but I've been a bit disappointed.