I purchased 1 of these as a test for my motorcycles - after a few weeks I purchased a second one so I have a few across my bikes. The devices are small, robust and easily hidden within the motorbikes. Very happy with the results. The STMT device uses roughly 8mb per month so very light data use. You might find that your Vodafone plan already provides with a spare SIM anyway...
There's a couple of gotcha's to be aware of...
1. Initial connection to the battery may cause some drain - so make sure you have a battery tender connected after install overnight.
2. There is an ongoing batter drain so if you're riding infrequently, you will need to have a battery tender attached ...if you ride every 3-4 days, that should be ok - noting each bike will be different.
3. The Phone application works better that the web site but you need both for the initial configuration work.
Very good value, performance and ease of use.