I have a DS1515+, and honestly, the only thing I use it for is file storage, and Plex. I do not have it exposed to the outside world. I am really tempted to stay on DSM 6 for as long as possible.
I also have a DS212J that I use for backing up the 1515+, That one is still on DSM 4 - as anything newer and the speed of the SHR drops by 50%.
I read the DSM 7 uses quite a bit more resources compared to DSM 6. I would almost want to perform some kind of RAM update first before jumping to that. I don't know, just rambling