Yes, replacement shakes are safe if they are from a good brand. Diet shakes are particularly useful for jump-starting weight loss (health experts generally recommend limiting weight loss to no more than about 2 pounds a week). As with meal replacement bars or low-calorie entrees, diet shakes help you keep tight control on calories. They are particularly useful for women who have very little leeway in calorie intake and who can't lose weight when they exceed their calorie quota even by a little.
Of course, diet shakes of any type won't work toward weight loss unless you eat fewer calories than you burn every day. To use diet shakes and meal replacement beverages most effectively, determine a calorie allowance for weight loss.