I still prefer Tumblr to Posterous. Themes are big. The user interface is big. Posterous has neither, and it suffers for it.
It's more that Tumblr's not perfect anymore. With v4, I couldn't think of anything I wanted added. It could have stayed like that for a decade and I'd still use it. It was fast, concise, beautiful, uncluttered. v5 is a step back. It adds clutter where there was none before.
I'm hoping it's temporary, because Tumblr's staff is incredible and they listen to users. (I hope more than just me complains about this.) But for now, it's pretty irritating, which is to say less than perfect. It's still the best on the market.