I took 2 different bus connections from and to Pattaya:
- Pattaya (11:30 Sukhumvit Rd at Pattaya Klang intersection in front of the little Thai Cafe Restaurant there) to Trat (160 B, 4-5 hrs)
- Trat to Border (every 20min, 120 B, 1 hr) to Koh Kong
- Koh Kong to Sihanoukville (dep 8:15 or 12:15: 4-5 hrs, $8)
Tipps:
- As noted, the last bus to Sihanoukville already starts at 12:15 from Koh Kong, so you will have to stay a night in Koh Kong unless you take a taxi (from $20) or hitch-hike
.
- It is strongly recommended to apply online for the new Cambodia e-visa at least 1-2 days in advance to not only avoid the hassle and overcharging at the border (pay $25 instead of 1200 THB) but also to save one full precious page in your passport (e-visa: 2 stamps; no e-visa: 1 page visa sticker plus 2 stamps)
I always liked the charm of this little town and have great memories about it, but it's pretty boring there during daytime (though it's cool to make a bike trip to the waterfalls now with the new road which is better than most roads in Pattaya!


(the new road linking Koh Kong and Phnom Penh / Sihanoukville)
SIHANOUKVILLE
I was pretty amazed by all the new roads they have built across the country, and now they have traffic lights also! (I have read though that since their installation, the accident numbers have raised sharply because almost everyone ignores them).
Everything felt a little bit more 'developed' since my last visit about 2 years ago - nicer shops, direction signs, the Baht lost against the Riel, you name it.
Great beaches as usual, low season so less hassle, many nice restaurants and bars, Victory Hill seems to turn in to a redlight district (2 yrs ago there were just 2 hostess bars, now there are about 10) with most girls looking like regular Yabaa users, great and affordable food at most of the well-known venues in town. I had the best "Fish & Chips" of my life so far in a small shop in front of "Utopia" (which is located close to the Golden Lion roundabout) for just 2$.
Bus trip from Sinahoukville back to Pattaya: I tried the 'direct connection' from Sihanoukville to Pattaya which can be purchased at hotels, guesthouses and a few agencies on the main road for $24..$28 (600-800B, 11 hrs). The best price of course directly at the agencies. In fact the trip is splitted up: Large bus starting, then switching bus at Sre Ambel bridge (1/2 hour break), finally switching into a minibus at the border with offensive and aggressive Thai touts who try to extract 'tips'. "Welcome to Thailand!"

Hope this is of some use for someone,
R