Being a cheapskate, I got the one of Rick Oleson's screens for my 2.8D. It was only $30 and is bright enough for me to use the camera indoors, which I couldn't do with an accuracy with the original screen. I've never used a Maxwell screen but for the price difference I'm happy to go on in blissful ignorance with my Oleson screen.

There's a whole host of different grid options and international shipping is free: