I upgraded from an E1, and the extra features are great.
When I purchased it I sort of did so to future-proof my self. I knew I'd be getting more and more headphones over the next few years, but I didn't want to buy some huge desk setup to drive them. As has been said before, this is the Swiss Army Knife of the Amp world.
It sounds amazing. Maybe slightly warm (which is better than sibalent in my book), but otherwise flat.
Some things that won me over are a 10-band EQ, mic on the unit itself, bluetooth, Volume Normalizer, functions as external battery in a pinch, and true surround decoding as of firmware update. Crystalizer and Bass can sometimes be fun for low quality and movies respectively. The sound blaster panel works on my computer and on my phone which is cool.
I struggled a bit between the G5 and E5, and am glad I went with the E5 now that they updated to give real virtual surround decoding (5.1 signal -> magic -> 2 channel headphones surround) rather than just stereo to "stereo sounding more surroundy" (2.1 signal -> guess what sounds should be surround -> 2 channel headphones "surround").
I did a fair amount of research to figure out what chips were in the E1, E3, and E5 before purchasing, here they are:
DAC Amp SNR
E1 TI PCM5121? Maxim MAX97220 106 dB
E3 TI PCM5122 MAX97220 112 dB
E5 Cirrus Logic CS4398 TI6120A2 120 dB
The G5 has the same chips as the E5
They all output 24 bit, though the E3 can do 96kHz and the E5 can do 192kHz. Why anyone would want this though is beyond me, like maybe two people ever can hear a difference between 44.1 and 96, and no one can hear a difference between 96 and 192. Plus nothing is mastered at this anyway.
One little annoyance is that both of the front jacks are only 3 band 3.5 headphone outputs, where on the E1 one of them was a headphone/mic combo so you could plug in a 4 band headset. The E1 also had an option in the Panel to use the 4 band one as a mic and the other one as the output which was good if you had a headset that terminated in two plugs - with the E5 the mic needs to be plugged into the back.
~~Another annoyance is that if I plug the E5 into my phone and use it via Usb OTG, it continually charges the E5, draining my phone battery. So it's best to use the E5 with my phone via 3.5mm cable or bluetooth.~~ UPDATE: This can be turned off by double tapping the power button.