The most common complaint is that using the mouse is slow compared with cursoring around, whether via arrow keys or via hjkl (in vi, etc.). This simply isn't true. The mouse seems slow but is actually faster:
The basic summary is cursoring around required a higher level of mental planning to organize the interaction, which apparently obscures the perception of the passage of time--think of being deeply engaged in something and being surprised when you look at a clock-- whereas the use of the mouse was done at a lower, mechanical level that left the mind free for higher things, such as complaining about the mouse.
One common complaint is that moving your hand from keyboard to mouse and back takes time and interrupts typing. This is true, but it doesn't take as much time as you think. Especially if you're using a keyboard without a numeric keypad, the mouse can be close by. With or without a keypad, eventually you get to the point where you don't need to look for the mouse. Your hand always leaves it in the same general place and automatically goes there, often in preparation for a mouse operation while the other hand is still typing.
It is true that it is slower to use the mouse for, say, deleting a tab from the beginning of every line than it is to use the keyboard and type "^xjxjxjxjxjxjxjxjxjxjxjxjxj" in vi. But at that point you're basically programming the editor (with a manually unrolled for loop) more than actually editing.
When the mouse is properly accelerated, many of us find that it's faster and easier to highlight the lines in question and then type and execute Edit s/^<tab>//g in acme or just type s/^<tab>//g in sam's command window. This is such a common operation that acme provides two shell scripts so you can leave |unind and |ind in the tag of your window and click on them whenever you want.
Notice the difference between acme or sam and (say) vi in running editor commands like search and replace. In acme you can just highlight the section you want, type the command, and you're done. In vi, you have to cursor to one end, mark it, cursor to the other end, and finally type the command. The cursoring takes much longer than the mouse. Time yourself.
In the experience of many Plan 9 users, using the mouse in Plan 9 for an extended period of time and then going back to using vi in Unix highlights the amount of time you spend watching the screen as you cursor around with hjkl. Having broken out of the hypnosis that Tog describes, I just get frustrated beyond belief. Yes, I am watching the cursor move so I should be occupied, but all I can think is ``damnit, if I could just click where I want to go I'd be there by now.''
Another point in favor of the mouse is that it is more expressive. Notice how the scroll bars work in the Plan 9 text buffers: left or right clicking to scroll moves the window proportionally to where you are in the scroll bar. It would be much harder to do this in a cursor-based system. Cutting and pasting in acme and rio via mouse chording is much faster than the equivalent in cursor-based systems, especially if you are moving blocks that aren't line-aligned. Time yourself.
It's important to have a good mouse, of course. It needs three real buttons, not two button with an intelliwheel in the middle. Plan 9 makes too much use of the middle button for the wheel to suffice. You'll just get RSI in whatever finger you use to click the wheel. Logitech makes good three-button mice. Most people seem to prefer the triangle-shaped ones over the oval ones. They used to sell for $50 apiece. Now you can usually find them for $5-$10 on Ebay.
The most important point is that you need to try using the mouse for a week or two before you complain about it. You'll probably end up agreeing with us.
You feel more efficient using emacs et al. because you're always doing higher level cognition, while acme and sam might feel slower since you're acting instinctually but you're actually moving faster.
When one makes a mistake in a search string, in emacs one uses ctl-S and ctl-R, it's a pain to correct the mistake; the "Look" command in acme allows you to just edit the tag and rerun the search.
The following is a list of mice that users have reported to work well with Plan 9:
(updates to this list are welcome!)
This page needs much clean up, feel free to help with it.