If calibrated correctly where you actually measure your wheel, a bike computer should be as accurate as a GPS.  It is not reliant upon seeing the Sats, so it will still be accurate when riding through dense trees, between tall buildings or through tunnels.  Many bike computers will provide info more relevent to a rider such as cadence and if your pace is above or below your average.  Top dollar units will provide altitude readings and work as a heart monitor if your into the training aspect of riding.   Also, bike computers, even wireless versions go several months between needing  new batteries.  The time before a GPS needs new batteries / recharging is measured in hours.

That being said,   I also use a GPS when ever I am going on long rides.  Especially if I might be going to areas I am not real familar with "which is often"   I use a Garmin NUVI that, though mainly designed for Auto use, has Bicycle and Pedestrian modes.     I have been meaning to look into some of the new full color screen bicycle models that have come out.     It would be great to have the voice turn by turn instruction for a specific route, such as on maps given out on clube rides. 

After several years of using Cateye and Polar cycling computers, I purchased a Garmin 305 last December ('07).  I have found that the Garmin is extremely accurate for mileage, keeps present and average speed, heartrate and cadence.  What's great is that, after my workout, I just plug in a USB cable into my computer and I can transfer all of the info into my training log.  The cadence sensor is also a speed/distance sensor that you can use in lieu of the GPS satellite.  This is great if you are going to be riding in wooded areas or if you are indoors on a trainer.  I love that I can use the same computer for running and cycling.  I have never had the battery die on me.  My longest bike ride is 6 hours, so the battery has never been an issue.  When not in use, I keep the computer on a charger in my garage next to my bike.  If you are an ultramarthoner or 24 hour endurance athlete, then the battery could be an issue, in which case, the Garmin 205/305 would not be a good choice for you.

I recently went through this same debate.  I was really intrigued by the new Garmin 705.  However, after reading several different forums, I decided to stick with a cycling computer.  There are more posts about bugs on the Garmin than I could count - a lot of people having the exact same problems.  Garmin has come out with fixes for a few of the problems, but it's still a very buggy device.  From what I've read, the GPS altitude functions are not very accurate, so it uses barometric altitude measurements to determine atlitude.  That's fine, except you can't recalibrate the barometer, so it gives poor readings.  The speed has similar issues - GPS not always accurate or not able to receive a signal, so it defaults back to a speed sensor, which is what a typical cycling computer uses.  If the $600 GPS functions aren't reliable and use the same technology as a $50 cycling computer, then why pay the extra.

I ended up buying the VDO Z3.  It has most of the function of the 705 (heart rate, altitude, speed, cadence, power, time, temperature, etc), and also interfaces with your computer, so you can download workouts.  It does not have a GPS though, so it won't find routes for you.  It does allow you to recalibrate the barometer.  I'm very happy with my Z3.

You are absolutely correct, BikeGeek.  My apologies.  The vast majority of people on forums are trying to trouble shoot problems, so it appears everyone who owns one has problems.  I'm sure you are correct that the majority of people that own a 705 are pleased with their purchase.

What I meant to emphasize in my post, and obviously failed to do, was that the altitude and speed on the 705 often uses a typical cyclocomputer speed sensor and barometric altimeter, and not the GPS signal to determine those metrics.  Therefore, unless the route finding ability of a GPS is a prority (admittedly a very cool feature), it may not be worth the extra cost.

One - somewhat serious issue with Gramin Edge 305 seems to be that it switches off randomly.  After somebody suggested that it is due o bad design of battery springs that is prone to lose contact if unit is sibjected to stronger vibrations I tried pushing the headlight bracket against Garmin to minimize vibration (Edge own mount allows small but non-negligible amount of play which contributes to vibration problem). After I started doing it the random switch-off problems disappeared.  This really was the only issue I ever had with Edge 305.

Woland,

My 305 shut itself down on occassion, but Garmin was happy to replace it - hasn't happened since.  The 705 has a few more features, and as soon as I've got the extra cash to pick one up, I'll be upgrading. - can't beat it especially now that it can read in from other AMT devices such as a power hub and give some serious infomation on the ride and metrics it took to do it.

Substituting a rare earth magnet for the original one is one solution to that dilemma. The rare earth magnets are considerably stonger and as such aren't as suseptable to "falling out of allignment" as long as its within a zone that is about 3-4 times larger than with the commercial magnet it came with.  Buy a package and just share them with your local friends that have the same issue.