nothing wrong with the single player. i liked it. kind of outdated, but you can install sikkmod on it, to make it look more up to date.
multiplayer is dead as fuck, pointless to buy if you want multi.
also a friendly reminder: dont buy any of the quake games from steam. q1 and q2 has no music at all. its a pain in the ass to make it work. (get a disc, rip music, get a sourceport which supports music)
man, there's so much to love in q4 sp, and there's so much wrong in it at the same time...
the gameflow, for the most part is ok, but it's painful to see a downgrade in many aspects compared to doom3 (which came out much earlier).
I remember myself blasting with quite a satisfaction through d3 (except for the ending), and then I played q4 sp - what a joke in graphics/world detail/level design/ - i repeat - compared to d3.
As a standalone spherical fps in vacuum q4 delivers pretty well, but when you know what game designers and the engine r capable of - u know they've fucked it up from the first few seconds on Stroggos.
the plot twist near the end of the game is kicking some life into the sad shoot/reload corridor action, but other than that - one cannot recall any emotional moments from q4.
d1-d2 r giving u spine tickles even nowadays, having those geniously crafted levels, monsters, bossfights, etc., along with q1, q2, hell - even q3 sp tier climbing gives a number of solid exceptional bursts of adrenaline and endorphines in the process of fighting through, especially when u complete it for the first time round)).
But q4? Nah... lacks a lot of fun&joy in comparison to any of it's precursors.
Its not worth it buying on steam (along with some of the other Quakes) because there mostly buggy when played on steam (and If Quakes 1 or 2) its lacking the CD Soundtrack that it came with originally.