I choose the user perspective, so I downloaded the client, install it, and then... well... then nothing: there is no one to talk to, there are no smileys, emoticons or shaking windows, no sex appeal. Ok, I can see my gmail contacts there, but hey, they already have a IM (MSN, Yahoo or ICQ), 90% don't have a gmail account, and they don't understand a nut about Internet, they just use it. Finnaly, the voice quality is worst than others, like Skype and VoIP Buster, so, there is no reason at all for me to change to Google talk.
Guys, in the IM world, people don't care about technicalities. What they really want are those sissie features like a big face showing you the tongue, huge lips or annoying shaking windows. A success IM client must be specified by a marketing department, engineers don't have the skills to do it.
Remember Ericsson phones before the Sony joint venture: they were very good, robust and reliable, but ugly as hell. They were designed by engineers, for engineers. Market share, minimum. Now, add an excelent marketing department, some sissie colors and functionalities, and you get SonyEricsson. Do I need to say anything else?