A lot of the my clients don't have or use AutoCAD so what I do is create a .pdf file and e-mail it to them. They can make comments on the .pdf using Adobe Acrobat and e-mail it back. I can have AutoCAD and Adobe open at the same time (ALT TAB) back and fourth between the programs or just print out their comments and work from the print.
If you don't want to do either of those, you can also use the SNAPSHOT tool in Adobe Reader and select the page or area with comments. Then paste it into your AutoCAD like an image using (CTRL V). After you have made your edits, just delete the image from the .pdf file that you inserted.
Your client or reviewer doesn't have to open AutoCAD or even have access to it.
I haven't used the Markup or RMLIN options. We actually create a layer called _Design with a color of 142 (red) and make it a non-print layer. Anyone who uses AutoCAD and reviews can give instructions on this layer, but we mostly use it to document our design information in the drawing so 3 months from now when some one asked why you design it a certain way, your backup is right in your cad file.
I hope you find these ideas useful.