Well, I can't say I completely agree, but the lamp can be mecha bad, but also VERY rewarding. So it's kind of balanced, but in an extreme sort of way. The lamp is pretty much random.
Using the lamp has given me more downs than ups everytime I use it, it's kind of exciting just to see what might happen. I guess if you're one to take chances, then the lamp is for you, but if your not, then don't buy it (or take smaller steps and buy a chest.).
Other chance-prize givers you could try are the fishing pole (for sale and is given out by your grandparents, you can keep using this item until you get something like a HUGE FISH, points, etc.),the shovel (despite it's small price, it's not really that rewarding...), and the plant (it's not as rewarding as the chest or anything else. Although I advise that you do NOT use this when your tama is a baby.)