WOW!!! What a great training tool!

To be fair, I've only had the Edge 305 for a little over a year. The model I have came with a heart rate monitor and cadence sensor. So far I haven't found much I don't like about it. The operation of the unit is surprisingly intuitive and the manual (available in pdf online) is very clearly written. I've used Magellan GPS receivers and the Garmin interface is pretty similar. The installation of the Edge 305 wasn't difficult at all, although the cadence sensor took a little more time than expected.

The training software that comes with the Edge 305 is great. You can plot your cadence, speed, elevation, percent grade and heart rate all on one graph. You can see how you reacted to a particularly grueling climb and even see if your heart rate spiked too early. If you have a hilly time trial and you're able to practice on the course before hand. The data from the Edge 305 can help you see if you push too hard too early, and it can help you decide how you want to handle it in the real race. With the purchase of the receiver you get a software package that includes a training log and some specific interface tools for the Edge, so you can load courses and workouts to the receiver. You also receive a free membership to Motion Base which is basically an online training log with some features the CD software doesn't include.

The GPS features are great too. I like the internal altimeter; so you don't get a jumpy track log if you lost signal while going through a canyon. I haven't had any trouble maintaining signal in even thick tree cover. They say the Edge uses some super-sensitive antenna. I'd have to say it works!

The wireless signal from the heart rate monitor and that of the cadence signal are coded. Basically that means you don't have to worry about someone riding up beside you with a similar unit and messing up your readings. I've heard of people having strange (i.e. super high) speeds recorded while stopped at a stoplight. Supposedly, it's because of the induction coil in the ground that tells the light when to change. I haven't encountered any such problem with the Edge 305.

The battery is a lithium-ion battery that is supposed to have a life of 12 hours. Unfortunately the battery is sealed inside the unit and can never be replaced. I've had several lithium ion batteries start to have shorter lives as they get older so I'm very skeptical about the permanent battery. Personally I'd prefer to be able to carry a spare in case the one I'm using dies on an ultra-marathon event like Calvin's Challenge. I have noticed that the battery indicator is inaccurate though. A warning popped up after 7 hours and said low batter and the indicator showed no power left, but I ran it for another 4-5 hours without a problem. I've only had it totally die once after 10.5 hours of use. This seems to be related to the temperature though, since it seems to last a lot longer in cool weather.

The Edge 305 uses the satellite signal for determining speed. I've compared this to the bike computer that I was already using and most of the time the numbers are pretty close. After most rides the Edge reads about 1% less distance and a slightly different speed. According to the manual, if the GPS signal is lost, the unit will use the sensor on the back wheel to determine the speed (only with the Cadence sensor model). This means you can use it on your trainer and still get useful information even if you're in your basement. I don't see why the unit doesn't do that all the time though, since the physical measurement should be more accurate and precise.

I know some people don't like the Edge 305 because it doesn't have stored maps. Personally I just use a computer program to mark the roads and I just put a waypoint at every turn. That has worked out great for me and it doesn't add to the weight of the unit like additional memory would.

One of the nicest features is the Virtual Partner. This feature allows you to race your previous time on a course. You'll have to do the course once to store it, then the next time you can race yourself. It definitely makes solo training rides more challenging.


UPDATE:
I've been using my Garmin Edge for a while longer now and I've made some realizations I thought would be helpful. First, the mount is not very good. When you push the button to change the mode, you're pushing against the mount's locking mechanism. I did this and lost it at 20mph. Luckily, it ended up without a scratch, but now I'm always sure I securely hold it while I press any of the buttons.

Second, if you'd like to add waypoints and routes to the Edge you should do it through an external piece of software. In order to do it through the receiver you have to first visit all of the points along the route (which often isn't feasible) and manually store them. A Javascript application is available called GPSbable (easily found through Google) that allows you to build a route using an online map and upload it to the receiver. You can also Google "Garmin Course Creator" for a decent web based program. Garmin hasn't developed this kind of software for the Edge yet so running this software isn't always the smoothing experience. Directions on building a route are also given on the GPSbable website.

Third, I've been using the Garmin training software and I still think it's decent, but it's hard to get a good analysis of your workout with it. A better program is CyclingPeaks. It doesn't totally replace the Garmin software though. You'll still need it to organize your courses and workouts.