I got this toy for my daughter when she was 2.5 years old. She loved it from the start and still plays with it often six months later. At this age, she doesn't make the type of pictures that older kids would, but she likes arranging the pieces on the floor or on the included boards. When she gets older, I expect that she will like making pictures, too.
Our pieces stick well to the boards. The stick even better to each other, so when you make a pile of pieces when cleaning up, they stay together nicely.
I think any kid that enjoys stickers will have fun with Colorforms. A big advantage that Colorforms have is that you have the option of picking up and moving the shapes. Plus, you don't have to worry about Colorforms being annoyingly stuck to things like furniture and trying to scrape them off.
One good tip I got before buying this set is to NOT discard the extra vinyl between the shapes. You can cut extra shapes from this material - such as crescents (great for smiles on faces), stars, flowers, etc.
The only negative of this toy is that after six months of use, the corners of the boards are looking a bit frayed, but after that much use from a toddler, that is understandable.