There is at least one other thread on this. This is a posting of mine one one
I've done it and sucked/fucked myself with it... kinda wierd. changes the whole context of "go fuck yourself" from a scathing remark to a bit of a turn on...
I have done one a long time ago, and it is quite difficult! For several reasons, but the first being it is indeed not easy to stay hard for that long with your dick in a tube of cold goo. I got around it by putting on a really tight cockring, which helped.
The other issue is you can either put the tube on the floor and dip your dick in it like you're doing pushups, and hold it for 5 minutes without moving, or you can invert the thing on you really really fast, but it will be messy and you have to press it into you to avoid leaks, which also hurts a bit after a while.
Then when you pull out, the directions say to let the mould sit for 5 minutes, which is wierd because it's already set, but also the mould starts to loose it's water content which makes it shrink a bit. The end result is you end up with a dildo that is a bit larger that you actually are, which I guess to many is a good thing, but I felt kinda cheated. The point of doing this is to get an exact replica, not a slightly bigger version of me.
Finally, I think the kits (in my local adult stores at least) only come in two skin tones. And the rubber they use becomes very very hard, so the dildo can actually hurt because it's not flexible. Also, for someone my size (7"), they don't send you enough rubber to fill the whole mould, so you HAVE to use the mini vibrator component and dip it in to fill it up. And if your dick curves a bit like mine, it's kind of a bitch because the vibrator tube part is a straight tube looking thing, so my dildo had the tip of the tube visible at one point on the side of it. And i didn't want a vibrator, just a dildo. Oh and you can forget about trying to get your nuts moulded in with your dick too.
Last words of advice: ALWAYS use a condom with these clones. Some lubes discolor them, and they end up looking like they got used once and never got cleaned... Which is why I never use mine anymore. Just looks dirty even though it isn't.