We were there in January of this year and I probably wouldn't go again in January. Previously we were there in June and August and I would say the weather was close to perfect.
This year in January it rained some almost every day and we had 2 days, including our last day, that were almost a total washout. It would be sunny for 1/2 hour, we'd lay out our towels, and it would start pouring rain again. We dealt with this 3 or 4 times until we gave up trying to be out in the sun. We also tried to out-drive the rain one day by heading to the East edge of the island, up through the North Shore area, and down through the Dole Plantation area. It rained the entire day and in all locations. A lot of times the rain is more localized on Oahu.
I spoke with someone at the valet and he said that the last week had been a little unusual in regards to rain, but not unheard of for the January season.
The other thing we didn't like about January were the evenings were much cooler. On previous trips we would be out until midnight in our shorts and tshirts. Most evenings after sunset we needed long sleeves or a sweatshirt in the evenings.
I do realize that the weather patterns were a little goofy in January. We went to Sunset beach and it was closed due to dangerously high surf, so we may have had an exception, but January was not the perfect weather we've had on our other visits.