I know nothing. But my library has the following shit by them
1.Dots and Loops
2.Aluminium Tunes
3.Emperor Tomato Ketchup
4.Switched on Stereolab
5.Cobra and phrases group play voltage in the milky night
6.ABC music
I could get them all as it would only be £6 but I don't want to be overwhelmed. So which of these is good (not most definitive). I'll probably get 5 based on the title (for similar reasons I'll avoid 3).
Main Q - are they any good at all?

i'd reccommend
'emperor tomato ketchup' on the strength of track 7 'noise of carpet' alone.
i didn't realise how many albums they had until just now..
i like sterolab
i'm great, trust my jugement
I've just bought an MP3 player
and have been putting loads of my cds onto it and have been amazed by how much Stereolab stuff I have. I haven't listened to them for ages but they are wonderful - the first three records you mentioned are all great - Aluminium Tunes is I think one of the many singles compilations they've done - very good. ETK is good, more guitar based I think, from memory - Dots and Loops is more electronica - not sure about the others, 5 and 6 especially but Switched On is probably a very early singles compilation. All in all though they are all highly recommended - a classic band, quality control can be a little off sometimes so just jump in and get them all, see what you like.
I would get
Switched on, Dots and Loops, and Emperor Tomato Ketchup. This will give you a pretty solid introduction.
Great title aside
I'd avoid Cobra and Phrases for the time being. Didn't exactly deserve its 0/10 NME review, but not one of their finer records.
From that list I'd go for Emperor Tomato Ketchup, often listed as their essential record, don't let that put you off, its an amazing record with a few of their finest songs on it.
Also ABC Music which is comprised of their BBC sessions. Its the closest you'll get to a complete overview. Besides they were a fantastic band live, and bar bootlegs, thats the closest you'll get to vintage live Stereolab.