We bought this for our son (who is now four-years-old). He cannot do it on his own yet but it is a fun thing to do together and he is very excited to build the models (we are on our third model now, about half way though - it takes hours to build each model at his age so we don't do it in one sitting!)
The parts are durable (they will bend rather than break for example) so that is great for small children.
The only negative comment I have is that it is impossible to tighten the nuts/bolts sufficiently: when our son does it he doesn't have enough strength so if he tries to play with a finished model it falls apart in his hands and if I try to tighten them then the threads jump through (if that makes sense) and suddenly it is loose again. There is a sweet spot where you can tighten it just enough so it is as tight as possible before it jumps through the thread again but even doing that he only needs to push the car around the room once or twice before something falls of or it collapses altogether.
However, this does not seem to put him off at all, he just asks me to help him fix it and then he plays for a bit again.
And in the end he spends much more time building the models carefully and taking them apart after wards so the fact that he only gets to play with it a little bit really is not a big deal to him.
But I could imagine some children might be more end product oriented and then could potentially be a bit disappointed that the models fall apart when they try to play with them...
All in all it is a very good product though. My son is nowhere near ready for "grown up" meccano so this is a great start into the more serious construction world.