I think most on here will reccomend the refinas etc. but for what its worth the training centre I'm at uses a collomix mixer. It gets battered 7 days a week (bonding, drywall adhesive, krend, multi) with no grumbles and has plenty of power. The only thing I dont like about it is the whisk that comes with it as it throws the plaster up if you arent careful.
If you fancy a cheap and cheerful one then my evolution mixer from screwfix works a treat. Its sturdy enough but not sure how it will handle heavy site use.