I went last year, and I'm going again this year. Expect people to be throwing water, shooting water, scooping water, etc. non-stop from about 9 in the morning to at least midnight. You'd think that people would get tired of it after an hour or so, but no, they are at it relentlessly.
There are a couple of safe havens: all of the shopping malls that I went into made everybody empty out their water guns before they could go inside. Ditto for hotels, or at least for the one I was staying at. Three guys got on the elevator with their water guns all loaded up. They were nice enough to ask me if I wanted a spray, but I very courteously turned them down. I appreciate their asking me, and I told them that. The bars at Nana Plaza also will not allow any of that to go on, and they are pretty strict about it.
You can either try to remain dry, or go out expecting to get wet. I have a look that can stop all but the most foolhardy of people, but once you get wet, then you might as well hang it up. I stayed dry from the corner of where the Nana go-go bars start all the way to the first soi before I caught it in the face from a guy who was shooting at someone across the road. After that, it was open season on me. Wear clothes you don't mind getting wet.
If you are carrying a camera or smart phone, be sure to put it in a bag that will keep it dry. Some idiot never found out how close he came to losing all of his teeth, because he was splashing water on my left side. If he had been splashing water on my right side, where I was keeping my expensive camera in a non-water-proof bag, he would still be picking his teeth up off the sidewalk.
Either way, be prepared to wrench the water out of your clothes when you get back to the hotel room. Either they will be soaked from water being thrown on you, or they will be soaked from the heat of the day. There are no cool breezes during that time of year, so be prepared for that.
Oh, and a lot of businesses close during this time, so you might want to check to make sure that any that you plan on visiting will be open. If they have a website, check there. Most massage parlors and bars will be open, so no need to worry about that.
I didn't have any problems with pickpockets or ruffians, but always be alert.
That's all that I can think of right now, but I'm sure a couple of other things will pop up before too long. I hope this is helpful.
