Same as level one cache. It is possible to speculate that a larger level 2 cache may help with certain types of programs or certain types of data...but the end result is that the overall system performance is what matters. For instance... you might have a CPU with a very large level 2 cache, which means it can hold a lot more of the data and instructions the CPU needs. Being able to hold more data/instructions should make it perform better...but what if that same CPU has a rather limited memory interface and has to wait on the main memory a lot? Now that level 2 cache isn't doing you much good...and a CPU with maybe less level 2 cache, but a more balanced and faster memory bus, will outperform the one with the larger cache.
Hopefully this makes sense. If you are picking between two of the exact same processor architecture (like between two Core 2 Duo processors, one with 2MB level 2 cache and one with 4MB level 2 cache), you can probably assume that at least in some situations (moreso in the future) a larger level 2 cache is better. Is it worth the greater amount of money? That's a harder question, because in most cases level 2 cache seems to have little effect on real application performance.
Again, the best way is the real world benchmarks. Specs of any kind are very often misleading.