I live in the DC area and go to the Johns Hopkins CF clinic. It's about an hour drive from the city and the CF team is great! They have a full-service adult clinic that has regular appointments on Tues & Thurs afternoons. I also work full-time, and find the CF nurses are very responsive to questions/discussions outside of clinic appointments - so you don't have to make the drive for every flare up. For the past couple of years, I've done my IVs as an outpatient and gotten the picc placed at a local (NoVa) hospital rather going up to Hopkins and being an inpatient. And I get alot of labs done in the DC area rather than Baltimore (not PFTs though). Also, Hopkins is very active with CF research & trials and the clinic docs definitely listen to the patient in determining a care plan. I haven't tried any of the other area docs posters have mentioned, but I'd definitely recommend them.
Since you don't have a car, you may want to look into a Zipcar membership. There's plenty of locations - especially in the Dupont area - and it comes in handy for those couple of times you just need the car for a few hours.
Hope that helps.