So, with the exception of one (1) picture, photostream has been 100% reliable for me. As soon as I connect to WiFi, there is a brief pause, and all the pictures that I have taken on my Camera Roll on my iPhone, appear on my Photo Stream on my iPhone, and, basically at the same time, on my Macintosh (I'm using Aperture). Speed has been very impressive - if I take pictures while on WiFi - it's almost instantaneous - maybe a 10-20 second delay - which, given all I really care about is eventually getting the pictures off my iPhone, is total overkill, but I certainly don't mind the speed.
The picture I took this morning, still hasn't shown up in Photostream, even though pictures taken after it have, the picture that is missing is sitting on my Camera Roll, and I've been on an excellent WiFi Connection for several hours.
My general assesment of Photostream is slightly different than yours (because I've had a different experience) - it's _almost_ perfect, with just a single missing picture. If It doesn't drop another one, I'll be happy.
I would love to hear from someone else in the apple community, or, in the best case, apple tech support, to see if this is a known issue that is being worked on, and isn't anything to be worried about.