I've been drooling over one since they first came out, but I never considered using one on a mountain bike. On a mountain bike I want to find/stay on the right trails and then get back to my car regardless of where the trail spits me out. I couldn't care less about speed or power output, though on occasion I will ride with a HRM because so many rides start out up, followed quickly by Up, UP and UP!!!, and I need to make sure I am not putting out too much effort too early. If I'm pulling the girl in a trailer I won't bother with a HRM because I don't need a computer to remind me how much I am suffering.
Also, as I am very candid about, I have absolutely no skills on a bike and having a $650 GPS on my handlebars is pointless, let alone a $1,000 rear wheel with a Powertap. It all depends on your goals, but if it were me I would go with a used eTrex (or something similarly functional and disposable) and a cheap HRM with instantaneous and max HR (so you can see how hard you were working when you were seeing spots on that last climb), probably about $175 for both. Put the remaining $475 toward something that matters (new seat, shock, wheels, tires, etc.).
Riding on the road is a totally different story. It is much easier to get really lost on road than off (at least where I tend to ride) and wind up several towns away from where I thought I was. On road I actually would like to monitor several variables (speed, average speed, avg grade, max grade, speed over grade, HR over grade, power output mapped to terrain and HR, etc.) for training, long term tracking, immediate curiosity, etc., not to mention a GPS map of where I am and how to get back to the car/home. This is where the 705 is peerless and the battery life isn't so much a concern.
You might also see whether "there is an app for that."