Some of my personal experience and background concerning rolling stock kits:
I like them. I ran out of real estate on my layout, enjoyed building structures such as those offered by Bar-Mills, Downtown Deco, Banta, Wild West Structure's, etc. I then decided to start building rolling stock kits. I've done a couple Intermountain, four Labelle, and two Ye Olde Huff-n-Puff, with two more waiting. I'm modelling the early 20th. century, so the wooden kits work very well for me.
Cost: I pay more to build a kit than if I were to purchase a car from any of the manufacturer's. I'll use my most recent Ye Olde huff-n-puff car as an example. The kit was around $35.00 shipped. Not bad. The trucks (Lionel archbar, it's a "transition" car from Lionel style couplers to Kadee's.) were roughly $10.00 from a forum member, replying to my wtb post. I wasn't completely happy with the brown paint I had used on a previous boxcar, so I went with Weaver's scalecoat boxcar red. Figure about $12.00 for that, which I really like, and would recommend them. Colorado Midland dry transfers from Clover House, about $10.00. Hobby shop wood that was needed to complete the car to the prototype, about $3.00. $2.00 for the Kadee couplers. $67.00 for a 36 foot boxcar. I did it on the cheap. If I went through Lionel for the trucks, the would have cost my roughly $35.00 for plastic archbar. The cost adds up fast. I have the car I want, had fun building it, and I'm happy, but you can start to see where if you model the Santa Fe, it's almost as cost effective to buy from one of the major's.
I'm scratch-building a caboose right now, and I'm well over $70.00 with a pile of wood, some trucks, and a few detail parts. Still need paint and some more wood...
Enjoyment:
It took me about 3 months to build the above boxcar in my free time. I like doing it, and don't get in a hurry, as you can tell. I was really excited a year ago to purchase a MTH woodside 64 foot passenger car set. It took me about 15 minutes to get the cars out of the box and on the layout. They're too long for my prototype, but work well enough. 20 minutes later, the excitement had passed. A couple weeks later, they started to look a little "plastic" to me. 2 Labelle passenger cars later, the MTH cars look really "plastic" to me. I prefer the look of the Labelle cars, and it takes be about 6 months to build one, so the excitement (and frustration) lasts a long time, for a better result, in my opinion.
Conclusion:
Kits are fun. They're expensive, and they're available, sometimes cheap, sometimes not. Search Intermountain, Labelle, Walther's, Ambroid, Athearn, Ye Olde huff-n-puff, Mainline models, Williams, Weaver, etc. They're out there, and sometimes the only way to get what you want. The current Importers make some very nice cars as well. It just depends on where your interest in the hobby takes you.