The only thing I have against XLN is their key mappings are sooooooooo different than everyone else.
Not a show stopper but it makes using their MIDI with say Toontrack EZDrummer/Superior without the use of a customized XLN-to-Tootrack drum map.
But I do like their product. No better or worse than Toontrack (EZ and Sup), Slate SSD4, NI Kits, BFD2 and Jamstix because each product has it's use and that's why I own them all.