I've read that Ivy Bridge is 10% faster at the same clock speed.
It depends on the specific mix of instructions that an application uses, but the Instructions Per Clock improvements in Ivy are
more on the order of 4-6%. Now, if you compare the "replacement" CPUs (the Sandy 2600K vs the Ivy 3770k, for example) at stock speeds, the improvements are greater, but that's also because of clockspeed differences. The 3770k has a 100MHz advantage over the 2600k at stock.
The two biggest advantages of Ivy over Sandy are the integrated GPU (which is dramatically better, and is now within striking distance of AMD's latest platform IGP and low-end discreet cards), and the power consumption at load (the 3770k runs around 17-18W lower than the 2600k at peak).
If you already have a Sandy Bridge CPU, and you aren't using the integrated graphics (or it is meeting your needs as-is in GPU performance), then there is
not a compelling reason to upgrade. In fact,
it seems that Sandy overclocks a bit more easily, so if you plan to overclock on air, you might do
better with a Sandy. You'd be much better served by waiting for the next "tock" (Haswell). If you really, really need more multithreaded performance now than you can get on Sandy, you'd be well-served to check out a Sandy Bridge-E platform system instead (which has a whole host of other advantages, even though the design is "last gen" from the desktop perspective).
However, if you are still on an older-generation part (maybe a Core 2 or Nehalem based system), because you essentially get the benefit of both the IPC improvements in Sandy, and the power-usage improvements of Ivy, then it is a much more compelling upgrade. That's why I was looking to upgrade my Core 2 Quad based HTPC system until my Lynnfield server died and I needed to act immediately.