Koh San Road is somethin else. It is commonly known as the "backpacker ghetto". It's a couple streets of cheap and somewhat trashy vendors popular with young backpackers. You'll find bumping music and crowds late into the night... most of which are white travelers... many of which have dreadlocks, some lack shoes. This area is jammed with dirt cheap hotels ("mattress in a box" style), restaurants with colorful lights, hookah bars, travel agencies, bargain shopping, and massage parlors. It was fun to walk around and see it, but I wouldn't ever want to stay overnight in the area.
I am a massage junkie. I'll take one anytime, any day from Yuriy. The massage parlors on Khao San Road offer one-hour full body Thai massages for about $5 US (here in the States, it's usually $60+). What a deal! We decided to get one at around midnight after a long day of walking. The women massaging us were pretty rough and seemed to fall asleep periodically. The massaging would pause and abruptly start up again. Not the best quality, but you can't beat the price. I would be falling asleep massaging past midnight too. I don't know how they do it!
This is a fun little area to grab a bite to eat (restaurant or street food) and get some cheap t-shirts or souvenirs.