Er, how many channels? I've seen a second hand 48 channeler going for £750, so I'd say £500's about right for a 24 channel.
Anyway, the Sirius is good as a basic board, especially for theatre. It has limited uses for rock, but I've done some stuff with it before. It can take an incredible amount of punishment (except the DBOswitch, which is so breakable). For a budget board, that price is pretty good, and it's pretty useful for training and things like that. My 24 channeler's done me well so far, and I'd say it was a good investment, but be prepared to hire other consoles for bigger shows.
I been using a Sirus 24 for the past 6 years, and i've got no real complaints.
Ours was bought new, and its just starting to show its age, ie. sliders are going etc. Nevertheless, its taken a lot of punishment, and is still going, just about.
Like herr_highbrau said, its a good basic board, and I have found it to be very reliable. It does not handle smart lights well at all, but then its not designed to be used with them.
Its prety basic, but still allows you to do decent and fairly complex programming, although i've found u have to be careful with the GO button, as the contact starts to degrade as its gets older. Unfortunately, we've had many a missed que due to the button not responding!
It still allows you to do chases, which you can link up to the sound system and do sound to light with.
Unfortunately, I have no idea, as to how much you should pay, as it was bought before my time, but nevertheless, I agree that it should/will be a good investment!