The Nike+Ipod setup works fairly well, especially if you're already running with an ipod. IME, accuracy was +/-5%, though a route with a lot of elevation changes can mess it up. You don't need the special Nike shoes, just get a little pouch for the senor and secure it between the laces and tongue of one of your shoes. Beware that rain on your ipod could mess it up. I also used the garmin forerunner 205 (without the heartbeat sensor). It was great when it was actually working, but became a pain in the ass. It often wouldn't charge properly if you charged it via an AC Adapter as opposed to a computer USB connection. I'd find the next morning that not only had it locked up, but battery was drained. Also, I noticed power tended to run low after two hours, making it lose connection with GPS satellites. Not a big deal for most runners, but I imagine it would be really annoying for someone doing marathon training. Last time I tried using it, it no longer seemed to hold enough of a charge to get a satellite connection and haven't touched it in a while. If you go that route, I'd only buy brand new from a place with a liberal return policy.
Now, I just use
www.mapmyrun.com. I don't do much distance for now (prob 6 miles max), and already have a good idea what route I want to take for a given distance.
ps. I never used heartbeat sensors. I would control intensity by eyeing my split times. The forerunner had this really nifty feature where it display you and another animated runner on the screen that you program to run at a specific pace. You can tell how how far behind or ahead of pace you are by the distance between you and the runner.