I played on cake pretty heavily in 2008/early 2009. Played a whole ****load of rush when it became available on ftp. After BF played some on cake through late summer/early fall, then again some more late winter, but it's been minimal. Software is ok but not fantastic and definitely needs more input from a players perspective.
Couple of things off the top of my head:
Needs a timebank for ring games. Please god do this.
The lobby feels sluggish/clunky.
Better sunday tourney schedules (just better over all tourney schedules honestly, but hopefully an influx of players will help with that).
As someone else mentioned, resizing tables is something of a pain in the ass. Default positions of opening new tables also feels...awkward I guess.
There's more, but it's mostly little things. The problem is though that the little things add up. It starts at mildly annoying but eventually reaches the level of people who hate the software with a burning passion.
As a side note, Jason Somerville did some entertaining sessions on cake over the winter and posted them on youtube in a series called Eating Cake.
http://www.youtube.com/user/JCarverPoker/videos
I suggest this, because at the very least you'll also get an idea of what the software looks like, and how a very good internet player bitches about different problems with said software. On top of entertainment/education obviously.