I got this for my then 6 year-old last Christmas. This set contains the right number of vortexes, spirals, spinners, bridges and tower pieces to be able to make some pretty cool and complex runs without being overwhelming. If your child is on the younger side, and this is their first time with marble runs, they might need a little help. It's not that it's difficult, but when you open the box and just see a bunch of plastic pieces looking at you and an instruction book with no words it can be a little intimidating. Help your child with a simple marble run until they are comfortable and then they'll be off and running.
The plastic pieces fit perfectly together and you don't have problems with a too loose or too tight fit like you might have with a cheaper quality toy. The only 'tricky' pieces are the spirals. You have to use the little yellow guys to support them or they won't work. Also, these are the only pieces that bend, and if you don't have them at the right angle the marble will either not make it around the spiral because it lacks momentum, or it'll fly off because it has too much. This isn't a problem (just needs an adjustment), but an impatient child could get frustrated. Also the bases may not be too steady as your tower gets higher and higher. A flat, non-carpeted surface is recommended, although my son was able to build the basic runs on his carpet. They were a little shaky though.
Especially cool were the pieces like the vortex where you can watch the marbles go round and round until they finally drop into the hole (it's kind of like a toilet). The spinners are not vital to the construction of the run, but they are fun to watch. The spirals add a little bit of a challenge and introduce the concept of momentum.
I also like this marble set because it comes with plenty of marbles, and you don't have to freak out if one or two accidentally roll under the couch.
When your child is ready for a more difficult challenge, I'd highly recommend the Skyrail Marble Run Roller Coaster 250 piece set, which is also made by Quercetti. All the track pieces are made of the same material as the spiral and it allows you to create loops and other types of bends. You can see my review on that product as comparison. Although we haven't tried it yet, it'll be interesting to see if we can combine the two marble runs to make a "super" marble run.