Apparently Sony's putting out a motion-based controller in the spring, which is kind of ridiculous considering the existing pads have motion-sensing in the first place BUT!

Software Titles for Motion Controller from SCE Worldwide Studios
Look! Here it is! ->Ape Escape (Working Title)<- In case the thread title didn't make it bleedingly obvious!
Echochrome 2 (Working Title)
Eccentric Slider (Working Title)
Sing and Draw (Working Title)
Champions of Time (Working Title)
Motion Party (Working Title)
The Shoot (Working Title)
Tower (Working Title)
PAIN*
Flower*
Hustle Kings*
High Velocity Bowling*
EyePet*
* Motion control function will be added by network update, etc.

And the link: http://www.joystiq.com/2009/09/24/plays … 0-release/

As long as it plays like the traditional entries and they leave everything about the terrible Million Monkeys game far away, this is easily the game I'm most looking forward to for the foreseeable future.

Ain't that the truth.
Also, apparently they're going to release an RE5 Director's Cut that supports motion based aiming using the wand, whilst controlling your character with the REGULAR CONTROLLER at the same time. That won't end well.

On topic, I am totally new to this series, but people seem to like it, so which ones would you recommend I (don't) play?

If you're gonna play just one title in the main series, make it Ape Escape 2.  They've refined the gameplay to perfection from the first game without all the superfluous additions of the third one, the 60fps framerate and graphical presentation are rock solid (the sub 30fps of the first is hard to look at, and the third tends to have some unfortunate drops), and the music score by Koji Hayama is the finest of 'em all.

I still haven't played Million Monkeys, though.  Do we even consider it part of the main series?

I actually really enjoyed Ape Escape 3 for the win.

I liked the movie-genre-themed transformation characters, and I liked the option to be able to play as the little girl. Loli FTW!

Ape Escape 2 really was just more of the same, only it was on the PS2. But I really think they innovated with AE3.

I really wish they'd release the musical scores for these games...they released the OST for Pumped and Primed, but that was just a spin-off from the main series; why would they release the OST for a spin-off, but not for any of the main series installments?!