Within the Ballston-to-Rosslyn corridor, you will not need a car. If you expect to be traveling by air or train (either for business or pleasure) the Metro now connects to both major airports and the main intercity train station. So connectivity is actually pretty good--it's hilariously easier to get to, say, Paris from Arlington than it is to get to Leesburg.

The problem is that public transport quality decays VERY quickly as you get away from that Ballston to Rosslyn corridor. Buses on some of the suburban routes are scheduled for every 30 minutes--and cannot be relied upon to arrive at any given stop on time. If your bus journey requires a transfer, you can just forget about it--you're not going to get where you're going in anything like a reasonable time period.

Along the Ballston-to-Rosslyn corridor, especially moving east/west, there are multiple overlapping bus lines that add up to pretty good frequency. Of course there is also the Metro, which runs underground in this stretch. Headways/frequencies are down from where they used to be, but they're at about every six minutes during the morning and afternoon rush and every 12 minutes or so off-peak. That's acceptable as far as public transport. On the extremely rare occasions where the region gets crippling snow (more than a foot of snow), the Metro will sometimes restrict its operations to only the underground lines, and all of the Arlington stations remain operational.

A lot of the corridor is easily walkable/cyclable too. I live nearest to Courthouse Metro; but I routinely walk down to Rosslyn or up to Clarendon to run errands or just hang out. If I'm feeling lazy and I happen to catch it, I'll grab the 38B bus or the ART 55.

Transport north or south of the main Fairfax Drive/Wilson Blvd/Clarendon Blvd. corridor gets a little less good. Metro's Blue Line runs south to Pentagon and down towards Alexandria. I haven't really tried the north/south bus lines in the area, so I'll defer to folks with more familiarity, especially along Glebe Road (which will connect Ballston Metro to points South).

I have a car here in Courthouse. It is very much a nice-to-have, but not a must-have; I could do without it if I really had to. So my total miles driven per year is really really low.