Some will say the broom will do just as good a job and keep you fitter with it.
Personally I noticed an big difference from using the broom after buying a Sebo duo, since then have used rotaries and an Envirodri. The Envirodri is by far the best in my opinion but heavy so still use the Sebo for some bedrooms where less soiling is normally the case.
If you mainly do domestics I would say get a Sebo duo, if you do a lot of commercials and don't want the expense of the Envirodri then get rotary.