Don't listen to the naysayers: the Garmin bike computers are wonderful! If you really want to get fit, they'll give you tons of information that will help (assuming you pay for the HRM). I bought mine on eBay and paid $410 (computer, HR, cadence, map card). It was during the live.com 25% cashback program, so my net cost ended up being around $300. At that price, I feel that it's a decent value. At $650 or 700, not so much.
Here's the thing about the 705: it's a bike computer, not a GPS unit. If you really want GPS and maps, you'll find that the screen is a bit too small and the controls are a bit cumbersome. I bought the 705 thinking it might be able to do double-duty as a hand-held GPS unit when I go hiking. It might, but it's not ideal for that. If I had to purchase again, I might buy the Edge 305 rather than the 705. In addition to mapping, the 705 allegedly has a better screen and longer battery life than the 305. And it will display power data if you ever buy a PowerTap, Quarq, or other ANT+Sport compatible power meter. I find that the battery in my 705 will last for a week of usage (1-2hrs/day), I never use the mapping functions when I'm on the bike, and I'll probably never own a power meter. So I could probably get by with the 305 rather than the 705.
If you do decide to buy a 705, realize that the included base maps are terrible. If you actually plan to use the mapping functions you'll want to buy a bundle that includes the SD map card or you'll have to buy the maps on DVD. I would not buy a 705 without the heart-rate monitor and the cadence sensor.