>> What does one look for in a joystick?
Well what games you wanna play is important. A Joystick works for Flight Sims THE best, but also works very well for some racing games (such as Star Wars Racer...
http://homepage.mac.com/mdf_occ1/MacChamp/ if you got PodRacer). I've had some troubles with using a joystick on some games such as Descent 3, but if Flight Sims are what you crave I'll repeat, it is a must.
A Gamepad is great for those side-scrolling and Role Playing Games, and sports games. I've heard of some people using them for 1st person games (like Unreal Tournament) but I can't imagine it working too well.
If your looking to play 1st person games and those point n' click games such as Myth2, I suggest you get a slick mouse. A good multi-button mouse is a must for playing games.
>> What brand do you use or prefer?
For Joysticks, I'd recommend the CH products. My CD Gamestick 3D is very slick with Flight sims, and even has a throttle! Check out
http://www.chproducts.com/index.html.
For GamePads, check out MacAlly's iShock. It's exactly like a playstation joystick, very nice feel.CHeck out
http://www.macally.com/new/usb/newishock.html
What is/are input sprockets? I hear this term in Macworld or see it on boxes...
Input Sprockets are a cheap, but very good way to make universal drivers for joysticks/mice/gamepads to work in games. Easily said, if the Game supports Input Sprockets, your peripheral is also supported. This isn't ALWAYS the case though, sometimes the joystick/mouse doesn't support Input Sprockets, so check the box before purchasing.
One more note, if you get a USB controller/mouse be sure to check out USB Overdrive. It'll let you use ALL functions of that USB device in any app...
eg. I bought an intellimouse... it has five buttons, and while I used the intellimouse software, I was only able to use 3 buttons in games. I installed USB Overdrive, and got rid of the Microsoft drivers and now I can use every button in games. It's a VERY good utility. Check out
http://www.usboverdrive.com/
>> Do all joysticks support all games (or vice versa?)
Read Above
>> What brands/features should I avoid?
Cyborg3D joystick has had reported problems with OS 9 I believe, other than that check out gaming areas for more reports.
Mac Gaming Ledge is a great place to start at:
http://www.macledge.com
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mdf-occ1
Mad Skillz Homie Dawg