I currently own a 4-plex, and in the past owned a 16-plex. I think multi-unit property is a GREAT path to ER, and you can really mitigate the work required by following a few simple rules:
1) NEVER buy property more than 15 minutes away from where you live, and preferably live in the rental property yourself. I currently live in one of the units of my 4-plex, and when I owned the 16-plex I combined 2 of the units into a single large owner's unit and lived there. In the case of my current 4-plex the other 3 units cover all the building expenses (PITI, utilities, maintenance, etc) so I'm essentially living rent free, and my time investment averages out to well under 5 hours/week (in spurts). At one point I had no move outs and no maintenance calls whatsoever for 2 years straight.
2)Buy property in an area you'd like to live yourself, not the cheap side of town. At one point all of my tenants were either post docs or grad students.
3) Be very selective about choosing your tenants, and charge below market rents to encourage them to stick around. In the case of an owner occupied 4-plex (and smaller) the fair housing laws don't apply, so you can choose your tenants. Even in the case of the larger building I was able to come up with legal policies that weeded out potentially problem tenants (like no smoking, strict quiet hours, etc)
4) Multi-unit buildings (duplex and larger) cash flow better than single family homes, and have reduced maintenance costs (there's only one roof, one yard, etc). A single vacancy is also less of a crisis, so you can be patient and not jump on the first questionable applicant
5) Don't use a management company. They have no incentive to find "good" tenants or to reduce maintenance expenses. I give reduced rent to one person/couple in the building to take care of day-to-day tasks and do all the tenant screening myself. If I'm going to be gone on a long trip I make sure that no leases are expiring while I'm gone.