I was able to finish model in 6 hours. The most difficult parts were the round parts. I used different pen bodies to wrap around to form those parts.

I'll give it a 4 star because the instruction is not clear. The most difficult part putting it together is the top part and the bottom of the wheels.

Christmas gift for my husband! Looks complicated but also looks like a great challenge for him! Can't wait to see the final product!

difficult to put together.

Unlike other users, I had no trouble putting together the wheels and the body. The cow-catcher assembly was the most frustrating for me, and the final result is a bit crooked when viewed from the front. Be very careful how you grip the body in later stages, because the long thin rails along the boiler kept getting squished.
The very first step has you bending the curve of the boiler around a form, but doesn't give a size for it. Based on another reviewer's recommendation of a 3/8" mandrel, I used a chapstick, but it proved a bit too big. If you look ahead to piece/step #11, you can see the groove the curve needs to match up with later.
The instructions show a much deeper curve in the two arches that attach to the boiler top than is actually needed; look at the picture on the package, and the slots on the boiler.
The small wheel rims (#14) I curved with a 5mm mandrel, which after springback was perfect. I rolled the pieces around the form with the tab end under the slot end, then bent the tab up with tweezers from inside the circle. I used the body of a mechanical pencil to curve the larger wheels in the same way. Assembling the wheels was the most enjoyable part of the model for me!
Flat-ended tweezers and/or chain-nose jewelry pliers are essential. I also used wire cutters to snip the sheets apart, which helps in separating the pieces without bending"”especially the long thin ones. I cleaned my fingertips with rubbing alcohol before starting and several times during, but still left tons of fingerprints. Just wipe them off the smooth pieces with rubbing alcohol at the end.
I would give 5 stars if the directions gave measurements or better pictures about bending the various curves. All in all, a fun model and I hope the recipient will like it.