This is a review comparing two RS-232 to Bluetooth adapters, which allow connecting a modern Android device (running XCSoar) to a legacy logger / vario / FLARM.
Disclosure: K6-Team has donated a K6 Bt to the XCSoar project. We have not received nor requested a donation from Glidertools.
The K6 Bt is very expensive, it costs € 148 (previously: € 140). It comes with two RJ-45 sockets: one with a RS-232 interface, and the other for connecting power (5..14 V DC; you can supply both combined on one port). The Bluetooth credentials come preconfigured and cannot be changed.
The baud rate is detected automatically. That makes it plug & play: plug it between power supply and the FLARM, and it just works. Simple setup when you fly in different club gliders, it's installed within seconds.
There is a (documented) protocol that allows baud rate switching from software. That is very convenient for devices that need to switch the baud rate for bulk transfers (such as Volkslogger and Colibri; however that protocol is not implemented in XCSoar 6.1).
The VFBT-1 is somewhat cheaper at € 95 (I bought mine at € 75 a month ago). Due to high demand, it's sold out currently.
It has two RJ-12 sockets (RS-232, and the second socket can receive data from the former, but cannot send), a power cable (7..20 V DC). As a special gimmick, it has a USB socket which provides 5 V DC for Android devices, but its data lines are not connected, no NMEA output here.
The baud rate comes preconfigured, and cannot be changed easily (and definitely not in-flight). The manufacturer strongly recommends not doing it, because it may brick the adapter. There is no "factory reset" option. If you misconfigure it, all that remains is a useless € 95 plastic box.
Confusingly, the manual also recommends not to use the VFBT-1 for transferring "huge amounts of data", and implicitly warns that nonobservance of this rule may damage the device. That is a very surprising warning for a product whose sole purpose is transferring data.
Update 2011/11/14: Vladimir Fux of Glidertools told me the reason for the warning was that Colibris had their electronic seals damaged during a data transfer. That is a firmware bug I know well (unfortunately) - at my club, we had many damaged Colibris due to that firmware bug, but LX Navigation will not fix the bug. Let your money do the talking, and don't buy a Colibri!
Both products work, but both are far too expensive for my taste. The Android port of XCSoar created a big demand for these adapters. Big demand and low supply keeps the prices at a high level. Let's hope that mass production will allow manufacturers to offer these adapters at lower prices.
The major advantage of the K6 Bt is the ability to switch baud rates at any time. Wiring it to a FLARM in any glider is a piece of cake, thanks to the two RJ-45 sockets.
The VFBT-1 is cheaper, and provides 5 V for charging Android's internal batteries. The possibility to damage it by transferring huge amounts of data makes it a no-go for me. I cannot imagine a technical explanation for that absurd warning, and in my opionion, Glidertools should better drop it, becauses it can seriously damage their reputation (and not the hardware).
You can build your own Bluetooth adapter with kits like this one, for € 29.
XCSoar 6.2 will have drivers for the Android IOIO, which is a USB host controller that can be connected to any Android device, and connects up to four RS-232 ports to it.
Many new Android devices (such as the Samsung Galaxy S II) come with a USB On-The-Go controller, which could allow using standard USB/RS-232 adapters.