Unreal series is a flash sale. Is it worth getting for a singleplayer fan?

The single player is boring in the Unreal games(at least for me). It's better in multiplayer.

You can always go to Youtube and watch single player gameplay to see if it would be something you would enjoy and decide that way.

Yes, personally singleplayer in UT games is nothing too amazing but Unreal 1 and 2 have a great single player mode Unreal Tournament 3 not so much but fun can be had. Considering how many awesome mods are out there for these games replay value is high. I own every Unreal game out there in the series from consoles to pc been playing UT games for long time for that kind of price you can't go wrong with this pack :)

I loved just about all of the Unreal games I played (the exception being the only console one I played, Unreal Tournament for the PS2, because the controls blew).
Not nearly as much fun singleplayer, but still good enough shooters in their own right. I played Unreal Tournament for quite a long time solo before I could upgrade my computer (at the time) to 2003/4.

If you're expecting a great single-player experience, then no. If you're expecting a decent-to-good single-player experience and you know how Unreal plays, then yes (at least, for Unreal 3, and kinda for Unreal 2004). I don't think I've played through Unreal 2 yet, despite that being purely single-player.
The AI difficulties are tuned well and at normal/hard difficulties rarely seem bottish (their predictability is bottish, but the actual skill-at-killing-you isn't unrealistic).
But it's nothing terribly memorable. You won't get much more out of the SP experience than you would if you only did MP. Some decent characters, and some background story for the tournament, but nothing that will blow your mind. Not a bad story, just not one that begs to be told.

...plus the weapons and vehicles are fun as hell anyway.

When you say "I've already got all of the multiplayer games I ever need" it feels like you're not really a single-player fan, so it's hard for me to answer. The UT series is the only multiplayer series I've ever really liked and played, and I enjoyed playing against bots. When I played UT2003 online, in addition to playing against others (where I was mediocre but found some games that were on a decent level for me) I played some invasion, which is cooperative and therefore the skill matters less. All in all, as someone who doesn't play many shooters and doesn't like most multiplayer games, UT still has a place in my heart.