There's a test of the Tempest in the reviews section on the main page of the site - search for 'Tempest' in the reviews box and you'll find it.
My personal opinion is that it's a great bit of kit. Slightly too warm for most summer use due to the lining, but good wicking and pretty water resistant. Works well for stull like mountain biking and fast moving thought the hood isn't as protective as a full mountain jacket's.
But anyway, since you have it, give it a go and let us know what you think of it. No, it's not 100 per-cent waterproof, but it's well up for classic UK drizzly conditions and you can also wear it as a mid-layer if you want. Anyway, try it and tell us.