The NI stuff, in my opinion, is an over engineered mess, designed to force you into using LabView and really don't work with anything except their proprietary windows driver architecture.
Well, that rings true for a lot of my experience with NI, but my recommendation was based on writing a linux driver for one of the NI digital IO cards in a university class. I figured if we could do it in that class, maybe they're not all that bad?
So... I also used PCI based NI (Digital/Analog I/O) cards in the past (around the year ~2000), which then were supported by "comedi" (open source linux data acquisition drivers).
On the other hand, I recently got a few USB based I/O boxes from NI where no documentation on the USB protocol (or, god forbid, even open source drivers) exist.
Also worth checking out are MCC's USB data acquisition devices, which have open source drivers and provide digital IO and high speed differential analog inputs: http://www.mccdaq.com/usb-data-acquisition/USB-1208FS-LS-140...