What is the expected behavior of ifModified:true ?? v1.5

What is the expected behavior of ifModified:true ?? v1.5

This sounds simple (just try it, right?) but this is becoming rather confusing because there is no documentation explaining how ifModified is designed to work.  Also, I've seen postings on various sites saying that the ifModified is broken, blah, blah, yet a lot of them seem to be due to a misunderstanding of how it is supposed to work.

For several days a couple of weeks ago I was able to use ifModified:true and it would convert the 304 response code to a 200 and it would respond with data from the cache.  Beautiful (assuming that's how it is supposed to work and I didn't just dream it all up).

I came back to it and now the 304 is coming through to the success handler and the data is null, and a co-worker is replicating the results.

I am at the point of wondering if I am really supposed to be handling 304s myself or if maybe a new version of Chrome is causing problems, or what.

Is anyone having success with ifModified in version 1.5?  And if so, what are you seeing for behavior?

Thanks!

- Jon