I bought some Platypus bottles to use with my filter (replacing the Nalgenes I used with my Miniworks/Hyperflow filters), and haven't had any trouble at all with them fitting the filter. I got a 2-liter Platy to use as my Dirty bag when the Sawyer bag fails - and so far, I've had no problems with it, either.

Filling can be a pain if you don't have deep water or falling water to fill it; shallow running water usually only fills it halfway. But, with a total savings of over half a pound, I'm willing to half-fill the Dirty bag twice to get a full liter. (If I need to carry water, I can always use my pot to finish filling the 2-liter Dirty bag all the way.)