I don't post here too often, but this is a topic I can definitely sink my teeth into.

MacAlly iShock:

This is a pretty good USB gamepad, if you're looking for something not-too-pricey (mine was 25 dollars new, when it was released). For those of you out there who also own home consoles, you'll be pleased to know that the shape is essentially the same as a Playstation DualShock, with identical placements for face and shoulder buttons (including the square, triangle, circle and x icons). The D-pad is round, but functions just like you'd expect a D-pad to function: precise, and not too mushy. Two extra programmable buttons around the outer rim of the D-pad, and a third right next to Start and Select on the bridge. It's a durable little pad, but since it's made of clear plastic, it tends to get dirty and scratched up real fast. Your mileage may vary.

Now, for the cons. The analogue sticks are essentially useless, sadly: very unresponsive, and have a nasty habit of sticking. You'll most likely never use them, which isn't a particularly appealing feature on a controller. It's also no longer being produced by MacAlly, so the only way you'll be getting it is second-hand. Luckily, whenever I see this thing pop up on eBay, it's dirt cheap (even brand new).

MacAlly iShock 2:

Two words: Stay away. This monster is a very, very distant relative of it's slimmer cousin. You might initially be attracted to the fact that it has a rumble feature, but if you own a Mac, you'll quickly discover that absolutely nothing supports rumbling. All that does is add extra weight and size to the controller, which brings us to our next point: it's uncomfortable to hold. MacAlly ditched the DualShock shape and button icon designs (possible legal pressure from Sony?) in favor of something that's not only bigger around the handles, it has edges to boot. Not sharp edges, mind you, but after a few hours of play, your hands will definitely start to ache.

The analogue sticks are still pretty bogus, and they've ditched the two extra programmable buttons around the D-pad (only the one extra button on the bridge remains, and they've replaced the fairly low-profile buttons from the previous model with big, round, mushy ones). Speaking of the D-pad, the biggest reason to stay away is that it appears to actually be an analogue stick masquerading as a D-pad. Mushy, unresponsive, and possibly even a little misaligned.

It's ugly, it's heavy, and it's definitely not worth the 40 dollar asking price.