I am happy with it, I have watched full movies with it and enjoyed the experience. I am OC'ed. When I watch something on Justin.TV it has a few stutters on startup, but it smooths out.
My question for those who are having issues is, were you on WiFi or 3g?
I can use it on both, but WiFi is better. Also, if you are on 3g, how is your connection? Flash is generally data intensive, so you need a really good 3g connection for it to work properly.
I tried it on both WiFi and 3G. WiFi was, of course, better. My issue wasn't with the video per se, but the whole phone just slowed to a crawl.
Of course, I'm not a big video or Flash game kind of guy (other than hitting a few sites to test it out), so I really only use it as a utility. If they can improve the performance, great, if not, I'm not going to lose a lot of sleep over it.