quick msg from nyc -
driving to and from nyc itself is a real pain. i was stuck in traffic jam for over an hr yesterday and travelled only 1.5 miles. This is mainly because everyone ventures out of the city on weekends. however, driving IN nyc, specifically manhattan, is completely fine. much more relaxing than driving in central london, + it's quicker and I have to say it isn't bad at all. I found it to be easy and straightforward.
as for parking, there's a private car park on each road, and they charge from $1/hr to $15/hr!! (depending on location). I left the car overnight a few times and I was paying about $12 / night. not too bad.
as for parking on the street - if you can find a spot, it's free in a lot of places, regardless of the day. if it's not free, it's very reasonable (about a dollar an hr). parking for motorbikes is a walk in the park - it's free everywhere (it seems), and parking on the edge of the pavement isn't frowned upon.
driving and having a car or bike in manhattan is far easier than having one in central london. unfortunately, people drive like idiots here and so it's more dangerous. drivers are very careless and just don't appear to have any common sense a lot of the time.
my 250hp V6 rental was fun

. I'd certainly encourage anyone visiting manhattan to rent a car for a day or two to explore the surrounding areas (like long island or the state parks) and to try out driving in manhattan just out of curiosity - I found the experience great because it showed me just how hard/slow driving is in London, and just how diligent London-based drivers are in comparison to their manhattan equivalents.
one more thing: the road surfaces are far worse than those in london. In some places, bits of road are replaced with giant iron slabs that are vaguely cemented to the earth - these dislodge at times and one ends up wading around in a mixture of cracked tarmac, cement, building materials, potholes and giant cracks....