Not sure if you've used the Magic Mouse for comparison or not. But if you have, how would you rate the Trackpad against it? I've been using a Magic Mouse for years, but the Trackpad looks like it might be more comfortable.
OK, so this is "general" mouse vs. trackpad, because I don't like the Magic Mouse and use a Logitech MX Master instead, but I do have a trackpad.
I quite like the Magic Trackpad and have happily used it for extended periods for general use... However, there are a few things for which I find a mouse is just that little bit better - such as precise graphics, Logic Pro (for some reason) - and the trackpad is pretty useless for games. So what tends to happen is that I use the trackpad until one of those jobs come up, switch to the mouse, then never quite bother to switch back.
Certainly, when I was using a Macbook regularly rather than an iMac, the introduction of the Magic-style trackpad on MacBooks cured me from needing to carry a mouse around.
One thing I've found that makes the trackpad work better for me is enabling "three finger drag" which - for some reason - Apple have now buried under "accessibility options".
If you were buying a new iMac I'd suggest taking the opportunity to get a discounted trackpad, because it is well worth giving it a go. If you've already got a mouse then you can get the trackpad instead of another Magic Mouse for $50, and if you hate it you can probably sell it and get your money back.
...and if you find the Magic Mouse uncomfortable, you might want to take the opportunity to try some alternative mice (I'm resisting the temptation of launching into a Magic Mouse hate fest here...)