Definitely still in beta. Some thoughts about it for those looking for feedback as of 9/12/13.
Tested with 4 major browsers.
Tried it out for my first mine attempt with GLD. (2.57148125 coins with CPU and some nvidia card testing) Minimal timeout / lag.
Site is lacking some details you'd expect for an exchange, but they are keeping it minimal/clean.
Session timeouts seem long (have not tested length/session trigger). No forced logout. Required to lougout manually to avoid automagic login (best practices, nobody does it, still worrisome) ...
Lag/delays on various functions in the interface: generate deposit addresses especially; you have to refresh before they show up.
Mostly intuitive interface but could benefit from some basic walk-thru's / man pages / help system / faq for the less able. Obviously these are lower priority while they streamline the functional bits. Or needs more user friendly pathing for basic functions (mouse over/click help functions, clear page header or indication of what function you're on).
Displayed hashrates seems to be more up to speed now. Remote monitoring of your workers could use an upgrade IMO, but again, minimal is usually better.
No fees currently while in beta if this sways anyone into throwing something at it for mining.