It doesn't help that the Raspberry Pi foundation finally released an update after 2 years...and left the specs almost completely unchanged. Not even a badly needed speed bump on the CPU or an ARM architecture update that makes it less of a pain in the butt to support. Not even a RAM bump, even though many many apps (XBMC included) bump into the RAM limit on the Pi constantly, severely degrading its performance.
The situation is so ridiculous that overclocking is officially supported on their default distro (Raspbian) in the installer. It really helps too. Even a little 300Mhz clock bump makes the Pi feel twice as fast (mostly from the increased clock on the RAM). Unfortunately, in my experience the built-in "turbo" mode is generally over aggressive with the Core and GPU and you'll eventually get crashes when doing 3D games or hitting the USB controller (and remember, the networking on the box is all USB).
For what it is worth, if you have Samsung memory (Hynix sucks), I've had good success across several Pis with the following config:
arm_freq=1000
sdram_freq=600
core_freq=400
gpu_freq=333
avoid_pwm_pll=1
over_voltage=6
This setup plays Quake3 smoothly (mostly) at 1280x1024 and runs Chromium reasonably well if you don't have too much other stuff open (beware hitting the memory limit though, swapping on the Pi makes it nigh unusable--no more than 2 or 3 tabs open at once). Another caveat: The analog audio will be crackly with this config, use HDMI audio or remove the "avoid_pwm_pll" line and reduce core_freq to 333 to match the gpu_freq. I don't have any of the B+s to try it out, but given the extra couple of years they've had I expect them to overclock even better than the old model Bs. If you are one of those poor suckers with a Hynix Pi, you are probably going to have to remove the sdram_freq line as well, it just doesn't overclock for beans.