Getting to JFK in public transport might work for a domestic flight but i won't want to go trough NY's public transport with 2 full suit cases.
When I travel it's usually for business and the time savings of the Express vs. the Tube just makes it a complete no-brainer for me.
People often complain that it's a very expensive and very short train ride but they forget that the short travel time is a FEATURE.
The express is sometimes worth it, but it very much depends on where you're going; the express simply dumps you out at Paddington which does not have great onward connections. Whereas the Piccadilly line, being a tube line that goes right through the centre, can often take you directly where you want to go (particularly as a tourist) - Piccadilly itself, Leicester Square, Covent Garden, Kings Cross, ...
For business I figure if the privatized transport from the airport is 1/2 as long and not silly costs I'll expense it without guilt. At supposedly $99 from JFK it sounds legit for a short business trip. Granted they claim PMV, so YMMV.
Certainly 1 full suitcase?
Shirts, pants, socks, underwear, perhaps a suit or two, and a pair of extra shoes? I can easily fill a suitcase with that stuff.
Do you do laundry on your trip? That would certainly significantly reduce the quantity of clothes required. Now that all airlines are charging per bag (previously all airlines allowed two free bags) it probably makes more sense to figure out how to wash clothes abroad than to lug around a lot of extra clothing.
