Raqs are pretty basic and do not have anything of the speed and flexibility of a normal server (not too sure if this applies to the new 550 though).
However if you just want a small server that does very basic work at reasonable speed for a reasonable number of non-intensive sites and you want a simple control panel for admin and users then the Raq could be okay for you.
For example hosting your own business or running a small reseller business comprising of mainly static/flash/swish sites and no heavy databases then a Raq should be fine.
HOWEVER - generally speaking Raq prices are excessive compared with what you will pay for a standard server.
If you know what you are doing with a server then I doubt you'd be best off with a Raq, on the other hand a Raq is generally easier to handle for the inexperienced, (with maybe a little help!).
I am using my 2nd RaQ after migrating from a RaQ 3 to a RaQ 4 recently and I do not have any problems with it.
True, in today's contexts, the RaQ seems to be under-powered compared to the other systems and are a shade more costly too.
But my usage are pretty basic and since I am new to dedicated hosting, the RaQ is a great machine to work with. But if you are familiar with Linux and want to tweak around, probably it is best to go with the other alternatives.
But as a workhorse, the RaQ is pretty stable.
If you are planning on hosting numerous interactive sites, you might be better off with the higher powered machines.