If you guys want more details, I just got an E17 I picked up here in the FS section:
E7:
DAC was better than my onboard Realtek noticeably. The Bass Boost was a disappointment since only level 1 made any sense. The rest were just nonsense. Features wise, it has a sleep timer but that was it. Not feature rich, but it gets the job done for a cool 80 bucks. As a portable amp, the E7 was just a hair better than the E5 in my opinion. Not very good, but again at the time it was released, it was very versatile. It couldn't really power anything too demanding. Honestly, for the $80 or so, its a great device. The DAC works and improves quality noticeably, the amplifier will make your phones louder, the bass boost (lv 1) will add some oomph to bass light iems and cans like my RE-0.
E17:
First off, it looks nicer and loses the scratch magnet front surface. The DAC uses the same chip I think (WM8740) but surprisingly it sounds better...much better. Where the E7 had a colder feel and sounded kinda thin, the E17 is a touch warm and sounds very nice is it worth ~2x the price? Probably not, but it is at least better. As a stand alone amplifier, the E17 is very good. It has multiple gain settings (0, 6 and 12 decibels) and the amp is more powerful. The Bass boost and treble boost are very very nice since you can change in 2dB steps up or down. In effect, this means the E17 can be used with something like the RE-0 which needs a little oomph for the bass, and it works equally well to boost treble to make something like the Monster Turbine Pro Gold a bit more lively. Besides this new eq ability and gain, there is the almighty sleep timer (which I will probably never use anyway).
Summary:
So again you'll ask is it worth the extra cost? For me yes! Its better as a DAC, better as an AMP and very very versatile. It'll do well with cans or iems, sensitive and hard-to drive, bass light and dark sounding. When the E7 was released it was the definition of versatility; we can now safely say that the E17 has dethroned it in that department. In the value department, the E7 is still better in my opinion. Of course the E10 offers the E17 sound for the cost of the E7 though you miss the portable amp. Now please do realize all Fiio products are pretty cheap so they probably don't compare with things like the Leckerton UHA-4 or HRT MS II+, but of course if you're considering Fiio you're on a budget and things like those roughly 1.5x the price or more of the E17.
If you want Value: E10 >> E7 > E17
If you want Sound: E17 = E10 >> E7
If you want Versatility: E17 > E7 > E10
I know this thread has had a ton of "Doi you idiots need to use t3h search bar and g00gle u nubsauce" type responses. Hopefully this and the headphonia review answer most of the questions you guys have.