Cheap filters?
 

Is there really a big difference in the quality of the results using filter kits between the high end ones and the lower end or even generic ones? I've got some generic ones that came with something I bought on ebay -- they seem to work fine on my digital cam, but I dont have a lot to compare them to. Any experiences with the XL2? Also, do you need an adaptor to fit the filters to the XL2?

Name brand 72mm is your quest. The difference is when you bolt on a matte box and use graduated 4x4 filters.
Depending on what the "something" you picked up on ebay is and the size of the filters supplied, you might be able to fit these with step down rings. If these are smaller than 72, they are not useful at all.

The ones I've got are smaller, but I was considering going for the same stuff in 72mm... I'm actually just looking for basic polarizer filters and a UV filter... huge difference between generic and name brand?

you get what you pay for
 

Buying used is one way to go, just be sure that there are no cleaning marks, scratches, or pitting in the glass. It's best if you can inspect the item, or have the ability to return the item for a refund.

I suggest getting the best, otherwise you run the risk of degrading the image. You've spent $5K on the camera and lens, it would be a shame to put inferior glass in front of your nice sharp lens. Unless that is an effect that you are creating for your shoot.

I use Tiffen filters. You can also use Canon, Nikon, Formatt, B+W, Schneider, Harrison & Harrison. Glass is best, or Kodak Wratten filters, but they don't make a circular polarizer. Wratten filters are made of gelatin, are fragile and must be taken care of. I have a fairly large set for my stills kit. Wratten gel's accuracy is the standard to which all others are measured.

I picked up a 72mm Tiffen set for I think 70 bucks or so. It was a UV filter and Polorizer. Works great for me so far. I'd shoot for that. If you want the 4x4 matte box later on, that's cool, but for starters, I'd go with the Tiffen set.