I've never used one (never even seen one), but that is an amazingly low price for a new Garmin GPSR, waterproof or not.
The product detail page at the Garmin website (StreetPilot i3) doesn't provide any information about the European version of this GPSR. The vendor specific website (link provided in the first post, at the top of this thread) suggests that the device comes with UK maps only - meaning, no maps for the Continent.
But - for a rider who doesn't plan to leave the UK, and who is willing to either leave the GPSR in the pannier when it is raining, or put a very good quality plastic bag over it - it might be worth investigating. My suggestion is that prospective purchasers have a good close look at the device in a store before buying it.