The real issue with this trail is not the snow or the elevation gain, but that my calves aren't made of iron. Everyone knows what they're getting into when they take on this beast. It's brutal, it's icy for the last mile and some combination of Ice Trekkers/Microspikes/Poles are advised if not required for this time of the year. This trail is steep (1,500 feet per mile) and without any traction in the snowy sections, you're going to have a bad time.
This was my first time doing the hike and despite the sadistic trail conditions, it was a beautiful day and the view from the summit was one of the best I've seen in my two months in the area. The joke you hear about Mailbox is that there's a mailbox at the top so that when you get to the top you can send for help. It certainly feels that way by the time you summit, but you will feel accomplished for seeing it through, and rightfully so.
Bring more water than you think you need. You're going to sweat and get drenched through an intriguing combination of intense trail conditions, wearing too many layers at times and not enough at others, frequent crying breaks on the few switchbacks that do pop up and of course, the ever present possibility of rain/snow/hail/whatever else this region can dream up. I packed for a Mt. Si like trip and found that I had shorted myself on supplies. Even though the elevation is comparable between the two and the trail on Mailbox is shorter, that just means it's going to be all the more intense. So, be prepared. The only other caveat I would have is to just keep those white diamonds on the trees in sight for the last 1.5 miles. It's quite easy to get turned around as the trail becomes scarce in that section.
Either way, I would highly recommend Mailbox if you are looking for a challenging hike or are trying to discover how hard you can push yourself before your legs fall off.
TRIP SUMMARY: Difficult, but rewarding.
9:15 am: Start
10:15 am: Stopped to debate whether or not this was a real trail or a fake trail that seasoned hikers take people they don't like to punish them
12:15 pm: Arrived at the summit
12:40 pm: Headed back down
2:30 pm: Arrived at car
2:31 pm: Fell asleep in car
??? - Woke up in car and drove back to the city hoping it was still the same day