It depends what you want to use it for as to whether it's any good. To be honest, for the price these days it's already looking a bit outdated - heck I can pick up a tablet with a processor a few times more powerful than this one for around £10 more when you take shipping into account, and that includes a touchscreen and wireless (which the Pi doesn't have natively.) Because of the slow processor, getting applications running on it is a mighty faff - I've been involved in trying to get Greenfoot running on it at any reasonable speed, and the long shot is after many heavy optimisations it still doesn't really. That's not just restricted to Greenfoot either - try running any application in X and it takes *ages* to load and respond. Even the default web browser borderlines on unusable.
So if you're planning to use it for a normal desktop machine, I'd say forget it. But for many things it could be great to play around with - if you want to experiment or test something, it's a nice way of doing that, if you want to learn how to use a Linux shell then it's a great way of playing around on something you can't really break (because it entirely runs from an SD card.) If you want to use it as a home media client running XBMC, that's been heavily optimised for the Pi to the extent that it now runs and plays HD video pretty flawlessly, so it could be a very cheap setup in that regard too.
However, where I envisage using mine most is in robotics - it's a very cheap platform that, while is a slouch as far as running a desktop is concerned, is orders of magnitude more powerful than a traditional 8 bit PIC or ATMega chip. That opens up all sorts in the realm of things that just couldn't be done before - things like analysing images to look for obstacles in real time. It's also pretty good on the power front which further makes it a plausible choice in this regard, and with the GPIO you don't need to take up a USB connection to connect to an external chip if you want to plug in extra circuitry.
So yes, it is a nice little thing if you use it for something appropriate - but you can't just purchase one and expect it to be an all-useful box.