I wear my MX helmet on the street sometimes. It doubles as my summer commuter helmet, and I'll wear it if I'll be going offroad at some point (so I'm not carrying around two helmets).
I find the turbulence isn't so bad below about 50 or 60 mph. That's fine for my commute, which is all suburban streets. If I'm going to be going faster than that, taking the peak off helps a bit. That makes it tolerable for short freeway stints, but I wouldn't want to spend all day on the freeway with it.
Goggles sometimes fog if you're sitting still, but usually stay clear when you're moving. Some brands are worse than others.
A couple of other drawbacks on the street is that an MX helmet is very loud, so earplugs are a must. Another is that there's no weather protection, so if you hit some rain, you're going to get wet.
- Jet Jaguar
It's not an adventure unless you need a tent, a passport, and a leather glove for your shooting hand.