Strengths: Able to hear how far (and how fast) you've run at any time; Lance Armstrong and Paula Radcliffe congratulating you at the end of your longest (or fastest) ever runs. Integrates well with iPod and most shoes (provided you buy and adapter)
Weaknesses: You need an iPod and a shoe adaptor (for non-Nike+ shoes) to use it. Accuracy depends greatly on careful run and walk calibration beforehand.
Overall: Very useful for long runs or when the extra motivations is necessary. Be careful with the volume though when running alone or in the dark as it makes it difficult to hear cars and traffic that way.
Strengths: Lightweight (you hardly feel it when you are wearing it); number of laps that can be stored; ability to set intervals.
Weaknesses: Cannot pair it with a chest strap or speed/distance sensor
Overall: A reliable, waterproof sports watch for those into long runs and who need to keep a log of their times/laps. I believe a new version (but not oversized one) has just been released that works with iPods as well, thus increasing the product's mass appeal.
Strengths: Robust Large screen allows for plenty of data to be displayed (up to 4 fields simultaneously, allowing-e.g.-heart rate, pace, distance covered and time elapsed since starting the run all to be visible at the same time). In built mini USB port makes it easier to connect to a PC.
Weaknesses: It can be big and cumbersome for some to wear; the 2 length wrist straps can be too short and too long for the wrist; an elasticated, stretchable wrist band would have served the purpose better. The distance calculations are a bit below par, certainly not as good as on the 305, with a tendency to display 1mile approximately 10-20 yards before the 305 does so.
Overall: A useful, robust and reliable GPS monitor that can help you plan and execute runs to plan. The large screen makes the map facility more useful than the 305 too. If Garmin comes up with a way to add a foot cadence sensor and a heart interval (V-V)variabilty to this unit it would be hard to beat.