I bought a used Woodmizer LT40 hydraulic about 10 years ago with the same thoughts as you have. First and foremost, the mill is great, easy to learn how to use, and easily handled by a single person, although for any serious production an operator plus 2-3 helpers are needed.
Sawing for others? I looked into insurance to haul the mill and cover liability, also workers comp for employed helpers, and the cost was so high that not worth it unless making it a serious business, which was not what I wanted. I will cut for others if they haul the logs to my site and I cut, they do not help (liability exposure, probably low risk but big loss potential), and they load and haul away their green cut lumber. I will not travel with the mill. I will keep slabs without charge or they can haul them away. And I charge by the hour, not board feet. Since I want a life, I don't advertise, but some cutting on word of mouth.
For personal use, great. I hired a guy with a Woodmizer to cut up a big white pine I lost in a storm, over 1000 board feet of lumber out of that one tree, 18" wide rough cut planks which are now the floor in one of the rooms of our house. With that my wife said I needed my own mill, thus found a used one, she bought it for me, story told. So I cut from our own land diseased and storm damaged trees: (pine - white, red and jack), birch, red oak, aspen primarily. For own uses: 2 x 6 studs for home addition; joists/heavy stock 2 x 8", 10" and 12"; 1" x widths 6" to 18"; oak, birch and pine furniture; birch, pine and aspen paneling; miscellaneous.
Cost effective? No, but convenience level is extremely high. And if you have lots of trees, as we do, extremely useful to put some of those to use other than firewood. I maintain a stock of pine, birch and oak for personal uses, also sell some to local carpenters for finish work. Carpenters/builders won't buy construction lumber because not graded (building codes and mortgage company don't permit ungraded lumber).
Rough sawing is very accurate, and I have done rough sawed planking and paneling used as is; also have cut thickness down to 1/2" rough for planing and use as thin paneling (5/16" finish). Edging I use for kindling; slabs for the gasification boiler. Also built a solar dry kiln. Be aware: you need space to dry and store lumber.
Final thought: think this through carefully, do lots of research, not an easy or quick decision on this one.