It's entirely possible to use HTTP POST for this kind of situation. One thing to consider though, is that you should avoid sending too much information from the client to the server. It's not uncommon, especially for DSL connections but also for fiber and copper, that connections are assymetric. My parents have a 5 Mbit downstream connection but just 250 kbit upstream connection, for example.
One example is an AJAX site for collaboratively editing SVG that I helped with the scripting for. We found that some users (me among them) had strangely bad transmit rates even though they were sitting on very fast connections. The reason turned out to be that we were actually gated on upload speed and not download speed because we sent very much information both directions.