MR8HD - great uni
First of all, apologies to those I may offend. The answer to your question is - yes you can record a guitar directly into the MR8HD, and have no problems. This unit is a fantastic digital recorder if used properly. Meaning, onboard effects, preamps, etc. are truly cheap on ANY digital recorder; it's not where the money is put in development. Not by Yamaha, Roland, Tascam, or any of them. This, and other digital recorders are best when viewed as a standalone digital recorder, clean signals in to record, then do what you want to work with the signal after you have a good clean recording. Geez - for $400 what would you expect? As for others having problems with their units, you will find few, if any postings of problems with the units from those that keep up with software updates. Familiar with PCs? That's why you don't keep using dad's old Radio Shack unit from 20 years ago! MANY MR8 or MR8HD users have absolutely no problems with them.
There's too much operator trouble in too many postings these days. Sorry folks, but if the shoe fits . . . . . . Good luck and enjoy your machine. Just remember to only ask of it what it is capable of giving, and have fun.