I've cycled this many times. (It used to be my route to work). It's great ONLY on weekends - during the week (and particularly during the hours the bus lane operates on the freeway) it's horrendous. The buses pull into bus stops right in front of cyclists and the red minibuses just don't seem to see them! But yes, weekends is ok. And scenic/
There are several groups that bike on HK Island:
--SIRs, as another poster mentioned
--Dragons, which is a Tri club and has several weekday rides.
Both of these groups ride early. I mean really really early. 5:30AM-ish. By necessity.
I've never been able to get out of bed early enough to join them, and while they are very nice folks, they're pretty fast. So if you're not a speed demon, you may get politely dropped. I'm a very experienced but fat endurance cyclist and simply cannot keep up with them.
--Rapha Cycling Club also has a Sunday no-drop ride, usually from IFC to Shek O, leaving at 7AM (late by serious cyclist standards). This is now my go-to group ride since they do an excellent job with ride leading and sweeping. Fellow cyclists' skills are quite variable though - keep your wits when riding with them.
You have to love hills to enjoy biking on HK Island. There are no safe flat rides; safe to me means getting away from traffic, and any flat road has loads of traffic here.
I'd also say you need to have excellent bike handling skills to ride HK Island. I call HK Island cycling the "dance with death" for a reason:
--Roads are narrow and shoulder-less
--Road surfaces are terrible, especially on the south side. Potholes, huge seams that eat wheels, grates, etc.
--Drivers aren't necessarily aggressive, but are impatient and sloppy/distracted
--You'll be sharing these windy, narrow roads with tour buses, double decker buses, psychotic minibus drivers, and boy racers/Ferrari owners with more money than sense.
As others noted, you can take your bike on the MTR if you go to the first or last cars, and take off your wheel. Technically you are not supposed to use the station escalators; MTR grannies have been forceful with me on this before, forcing me to the very crowded elevators. Note also you technically are not allowed to bring a bike on the Mid-Levels escalator - I've been stopped doing that before too. Had to walk up from Hollywood to Conduit in my cleats. Boo.
Triathletes train frequently on the Sunny Bay-Tung Chung Road; while not perfectly flat, there are no hell grades, traffic is (relatively) light (watch out for garbage trucks though), and surface is quite good with exception of some oil patches. But it gets boring as hell after a while.