Unless you're competing (or planning to compete) regularly and already at the limit of the amount of time you can afford to spend on the bike then, for most people, I'd suggest they're probably not worth it. The cheapest will be something like the Watteam or single-sided stages for which you're still talking around the £300 mark - which for most people is a lot of money for a number which will be of interest / boasting value only.
If on the other hand you're regularly competing on the bike and are can't actually spend any more time in the saddle (without getting divorced/fired!) then they're a pretty cost-effective way to gain power (and therefore time), probably second only to getting a professional coach (and certainly more cost-effective per second saved than new wheels or a fancy skin suit
).
