I just got back from Sipidan (10/1/04 - 16/1/04) and the conditions were fantastic. I was worried initially about the timing as to when I went. Based on all the recommendations it spelled 'stay out'. BOY am I glad that idea was wrong.
Visibility always seems to be an issue, however in Sipidan I really don't understand why people plan their trips around it. Regardless of when you go, the visibility may be challenging based on the conditions at that time. Yes, EVEN during the recommended April/May season. Just quoting the divemasters. The monsoon season isn't ideal, but it's still very good there. I know that on the east coast of peninsular Malaysia it gets so bad that operators shut down, but that just isn't the case in Sipidan.
In my 1 week stay there, it showered once and during our surface interval too! For those not in the know, Vis will always be a problem based on the tides (receding tides pull silt, thus reducing the visibility). The visibility varied from 2m to 30m while I was there. Note that the depth of where you are does play a factor, and infact the best vis I got was during a Deep dive (while the worst was during a shallow night dive).
If you can, time your diving with what I call 'Ulli season'. Ulli is a German recreational diver lady that frequents Sipidan, and is legendery in her own right. She's done 1000+ dives there, and is the bottom feeder of bottom feeders. I'm a slow breather, yet she's about 30% more effiecient then I am! Anyhow, she really knows how to find just about anything there.
If you go, don't miss out on the Turtle Cavern dive (with a dive guide). It's amazing. Inside are flash fish which are the underwater equivelant of fire flies. The deepest dive I did was rewarded with a sighting of a Hammerhead shark(courtesy of Ulli). Rare around this time of year. Be wary of them jellyfish though... there are some nasties there that some people had a bad allergy reaction to while I was there. I even saw 2 Portuguese man o war jellyfish, rarely seen on the island. So cover your skins!
By the way consider your dive operators carefully as some are so much better then others. I REALLY enjoy walking 10m to a shore dive after suiting up. (others you have to walk a bit further and some even time their dives around high tides so they can get their boats to their resorts).
I'm going back!!!!!!!!!!!!