We were at Pearl last year in Jan. and this year we'll be back again in Jan. - college babysitters are easy to find right after New Years!
The temperature is certainly comfortable - the only time we ever felt chilled was walking back from the hot tub late at night (or early in the morning). Last year there was a pretty decent crowd - I'd say at about 75% capacity - which was just fine with us. We've been there the week before Christmas when it was at about 50% capacity and it felt a lot more mellow. Still lovely and relaxing but definitely a different feel.
They do spice things up a bit for ISM month. Some of the games are a little more risque but all in good fun. We won a contest where you had to simulate as many sex positions with your partner as possible in 30 seconds. It was silly and fun and we won water bottles and hats with the ISM logo on them. Needless to say we couldn't bring them home! And the foam party was definitely more wild than we had seen it in prior years. They have these HUGE inflatable unicorns that people ride all over the pool. We were a little concerned that we might not fit in with it being ISM month (we aren't in the lifestyle) but I don't think the ratio of swingers to non-swingers was all that different than usual. Just a great bunch of people in paradise. Really looking forward to going back!