Two guys I regularely ride with use the 205 and 305. Although the 205 was originally my GPS of choice since it had all the basic features I wanted, but didn't have the heart rate monitor and hefty price tag, I quickly changed my mind. On xc rides with both owners, the 205's elevation measurement was consistantly off by considerable ammounts, and if it ever dropped below a certain speed (like on very steep, long, slow climbs) then it stopped reading and the ride time/average speed/etc. were all thrown off. The 305 however, was far more accurate and overall seemed like a much nicer unit. It could have been chalked up to a faulty gps since I don't have numerous owners of the same gps on the same ride, but I have a feeling the 205 cuts a few more corners than just a lack of heart rate monitor for the pricetag.
My 305 has the heart rate monitor and cadence sensor - when it looses GPS it just gets info from the speed sensor on the wheel so i never loose average ride time etc, its all wireless too so very neat.
Not tried it on my DH bike but couldnt live without it now for xc and road.
I tried using the 305 a bit for DH but it doesn't read accurately enough for tracking DH runs. The fastest you can have it uplink is 1 sec. This isn't fast enough for a DH race track. Because it will sometimes loose GPS signal in dense trees or weather, it is hard to really use it as a tool for comparing times run vs run. You can't accurately see different line choices affecting times since the the GPS resolution is only up to 3 meters. Lastly, the standard mount is not solid enough for use on a DH bike. On the bar it is awkward and prone to being ripped off from a crash. On the top tube and you hit it w/ knee pads and you cannot access the controls to start/stop.
Overall, EXCELLENT tool for XC and road, just not yet for DH. I'd say either those moto timers or the watch/wands are your best bets for DH personal DH timing.