I always use the bus between NRT and central Tokyo. It's not fancy (like the deluxe airport buses at ICN) but it seems reliable and good enough. Given the potential for traffic jams--which the buses are usually pretty good at avoiding--it's good that more of the buses are equipped with lavatories. Upon arrival at NRT, the walk with luggage is about the same for a taxi and bus (unless you've arranged for a limo driver to meet you in the terminal) and less than what is required when you take the train.