Your enjoyment of this trip will depend completely on the weather. We go up to the summit when it was nice out - cold but clear - and it was a sight to behold. Beautiful, otherworldly, straight out of a sci-fi flick. However, about 10 minutes later it started to rain and became bitterly cold and windy. We still made the short trek to the permafrost lake (~35 minutes each way from just below the summit at a cautiously slow pace given the altitude) and spent the next couple of hours at the visitors' center trying to dry out and warm up. Unfortunately the rain held up so there was no telescope viewing that night. I consider my crew lucky to have made it to the summit before the storm. Most at the visitors' center had not been so fortunate. By the time we left it was 20 degrees F at the summit and the scientists were considering evacuation. Ack!
For some reason they keep the visitors' center bathroom doors open, making them crazy cold. Strangely, I recommend the porta'potties near the summit over the real bathrooms at 9000 feet.