I just went from a Denon 3803 to the X4000. It allowed me to run all my connections through my AVR via HDMI instead of sending video signals and audio signals separately. I was able to remove FAR more cables than I am currently using. It added Airplay and networking to stream content. Sound was improved with audyssey room correction. Also, it's more user friendly with the better on screen display, and the aforementioned room correction to setup the speaker distances automatically. Listing all the things that I bettered makes it seem like a no-brainer. It isn't. Knowing what I have learned over the last month or so by reading posts here as well as setting up and using receiver for a few weeks, I would be split between waiting another 3 months or so for the next version of the receiver or buying the receiver I did.
The decision as to whether or not to upgrade is certain...I would! The decision to wait for the next generation isn't as certain. What swayed me originally was feeling I received a bigger discount on this years model now than I would receive on the next generation in a couple months. I also will not replace my current TV until 4k TVs mature a couple years and price comes down on those as well. Several years ago, I would have waited to get the latest things and paid more for it. Now, I don't mind being fashionably late (but not absent) to the latest technology if I save some coin and let others work out the glitches.
So, as to your question as to when to upgrade. IF you want to keep your receiver through several generations and be reasonably current a few years from now, wait for the HDMI 2.0 receivers to come out later this spring/summer and receiver in your price range. If you aren't going to adopt 4k TV and Bluray in the next 5 years, wait for closeouts on the current models. I bet you'll be able to pick up a Denon X4000 or other manufacturer's upper midrange receiver for $700-800, and that would be a DEAL for what it offers.