The bike I'm riding now has stem shifters. I've only been riding it for about a month, and previously was using downtube shifters and I've not made up my mind just yet regarding them.
The advantage I see over the downtubes is that my hands don't need to move quite as far from the brakes to shift when they're up on the stem. Also, the shifters and their current positions is closer to my field of view than the downtube.

The downside is that it's a rather unatural angle to access while in a good riding position, but that may be just because I've gone from 10 years of downtube shifting so my instinctive movement is to reach for the downtube, remember it's a different setup and correct myself.

As to impaling yourself on the shifters - I think if you're slamming forward with that much force towards your handlebar stem you'd just as likely do a nasty number on yourself hitting the stem as you are hitting the shifters. I've never smacked the stem in a wipeout, so I think it's a moot point, personally.