Summary: Top of the range GPS, value for money.. Started to use a GT11 4 years ago, after 2 years I went to the GT31 which is in MPOV the most accurate, compact, versatile, waterproof GPS on the market. There is a bluetooth version but don't bother with it. It is also the No1 GPS being use in worldwide pro speed windsurfing. This GPS can be used anywhere in any speed, distance sports. I just bought a second one to be used in pairs with the other one.
Summary: The Locosys GT-31 is the best GPS on the market for speed sailing. It records your speed every second with Doppler accuracy. Easy to use, the battery lasts all day, and it stores enough data on the SD card so that you can sail for years without filling a 2 gig card. Other non-Doppler devices are not as accurate so can't be used for serious speed sailing. I use this for windsurfing, it is important to always carry the unit in a waterproof pouch.
Summary: A good bang for the buck, It has a lot of features, I love that it records at 1 hz. Cons, I wish it had a bigger display. It needs to be more durable in the water. Some how I tore off the rubber seal on the top of the thumb stick. It is susceptable to getting rubbed off by slipping it into your pocket or a duffle bag. Then it will leak water.
Summary: I bought this unit to use while windsurfing. The box indicates it is waterproof and that it is mainly used for windsurf. Well, it IS NOT waterproof after you open the cover a few times to access the card . It requires a waterproof pouch that you need to buy extra. If you don't, even traces of water will make it fail, as happened to me after using it less than 7 times, but as usual, just a few days after the warranty expired.
Summary: I have bought about 20 logging GPSs over the last 10 years for measuring very fast sailboats. The last 10 have been GT 11s and 31s. The GT 31 is, by far, the best I have had. It is sensitive so it starts up quickly, responds to changes quickly and does better with a partially obstructed signal. The battery life is long (I leave them on all day without problems). If you forget to charge the battery, you will get data on the SD card until the battery fades.