Space Quest is a great game series, methinks, and well worth the money. They are a different kind of adventure games than those of LucasArts - you will need a lot more patience, perseverance, and forgiveness if you want to get anywhere, and you really ought to have a notebook at your side at all times. The earlier games in particular make no concessions to the player; if you want to succeed in Space Quest I and II, you had better be prepared to work for it. In my opinion, it is worth the trouble though; the writing is excellent, and the zany humour is in a class of its own - it isn't for everyone, but I know I loved it. It is still fun to play through it time and again, though the latter IV, V, and VI have definitely become my favourite episodes in the series - in particular number V, which made up for its lack of voice acting and production values with some truly great adventuring, accompanied with a superb parody of Star Trek.
BUT: I wouldn't touch the Steam version if I were you. This is the same release as the one Vivendi put out on CD back in '06; it uses an outdated DosBox with horrible config settings. There is timing issues, sound issues, and stability issues all over the place. It is the worst DosBox job I have ever seen, and is not representative to the classic greatness that is Space Quest. The same goes for the Kings Quest collection, and the same will probably be true to Leisure Suit Larry and Police Quest, if those collections make it to Steam as well.
There is a fanmade patch out there that can fix all those problems, but it can interfere with the Steam DRM, so to be on the safe side, I would hold out until these games comes to GOG and are given a proper treatment.