I've been several times over the past fifteen years, though not recently. The Field's main hallway is how I've always envisioned natural history museums: sky-high vaulted ceilings with some spectacular exhibits. The elephant is impressive enough, but then there's Tyrannosaur Sue, posed at full sprint with jaws agape and claws reaching down....wowl. (I admitted to a docent that I'd always envisioned T. Rexes as larger; she just said "They're big enough.")
Check for special exhibits--the one I remember vividly is the Chocolate exhibit about 14 years ago. That said, the regular exhibits are fun for kids and those of us that just feel that way. Mummies, gems, fossils, the playlab, and so on.
A couple of disclaimers: I took public transit to visit the museum, so didn't have to deal with parking and I did NOT come on the 'free to Illinois residents' day, so the crowds were minimal.