Well, if you mean if it's worth the price as a *game*, at it's current state, then...
Not really, imo. It is still rather problematic in the multiplayer section, has a fair share of bugs, for some people their internet setup doesn't even let them play the multiplayer. And, past the novelty, it's still a combat-focused system, with very little (i.e. almost none) non-combat stuff to do just yet; and the balance isn't exactly nailed just yet either.
However. This does give you a good chance to see how the flight model feels, how the game plays on your computer, how the controls suit your habits, and it is an additional chunk of development money for the devs. And you get to flag up issues you find as part of the testing.
So, basically:
- If you want a playable game, then No, not yet. (Maaaaybe by the time alpha 4 comes out, but that's a large maybe)
- If you want to participate in testing a very unfinished thing, then.. Yeah, I'd say it's "worth" it. You won't get 100£ worth of value out of it, but if you can afford it and if you want to get in on the action asap, go for it. (Really, that's the underlying question: Can you afford the 100£ as a disposable sum?)