I have played with ANT a bit for my forthcoming micropower-Arduino project. I posted some very quick & dirty interface code here and an EAGLE breakout board for the pre-assembled modules, but got busy with some other stuff and haven't got around to making it into a library yet. That code does work though 
If you don't want to decode it on the device end, there is source code around for PC clients (such as libfitbit) that can probably serve as a starting point to interfacing a specific device. If you have a specific device in mind, check around in case someone has already done the work to reverse-engineer its protocol; may save you a lot of time!
So far I have only used ANT radios to talk between my own microcontroller boards (or microcontroller board to ANT USB radio stick); I haven't tried talking to an ANT+ (or any other 3rd-party) gadget. I also haven't looked much at the "+" standards, but I remember them being pretty well-defined for the published ANT+ profiles. If you can, dig up documentation for ".FIT file format" and "ANT-FS"; two related standards a given ANT+ device might use. You might have to sign up an account on their Web site or buy a development kit (or use google-fu...) to download those standards though.
As implied by the "not-quite-a-standard" rhetoric, if the device does not conform to one of the published ANT+ profiles (of which there are just a handful), including having vendor-defined fields you want to access, you will probably have to do some amount of device-specific coding for the particular ANT+ (sport, etc.) device you want to talk to. I don't know the rules on what gets to be marked as an "ANT+" device, but my understanding is that the data for any published profile device will be accessible by any ANT receiver using the 8-byte key defined by the profile. (Some ANT (non-"+") devices are on so-called private networks; to get anything from them you have to know - or reverse engineer - a secret 8-byte key. Fortunately these keys are a somewhat poorly-kept secret (google is your friend) for most of the well-known manufacturers, and many of those devices' protocols are reverse-engineered already.)