Bringing back the edge on an almost new chain by hand in not too difficult after a little practice, however it is much easier to get all the teeth angles consistent with a machine. I also have been looking recently at various types of electric sharpeners. I enjoy sharpening by hand, however my right shoulder, like the rest of me, is getting old and some days is reluctant to file as often as I would like, which is generally after 2 or 3 fuel refills.
There seem to be very variable reports on the various low cost machines that use a disc. Some people are happy with them, others think they flex too much, or are just unpleasant to use. One thing to watch for is that on many you have to clamp each tooth by tightening a screw, then undo it again before moving on to the next. That could turn chain sharpening into a real chore.
The sharpener that appeals to me most for sensible money and what I have ordered is made in the USA by Granberg PRECISION GRINDER (12V) - G112XT - Granberg International. It gets mostly excellent reports and there are some good videos on youtube that show it working
How To Sharpen A Chainsaw Properly - YouTube
Granberg precision grinder and accurate depth guage raker setting for chainsaw chain - YouTube