What’s Better: Assembled or Ready-to-Assemble Kitchen Cabinets?

Our Answer

Rest assured that there’s a wide variety of quality kitchen cabinets, both preassembled and ready-to-assemble, available at reasonable prices.  They're generally available from one of two sources, and the source you go with will determine whether or not they're assembled for you:
A big box store like Home Depot stocks quality preassembled cabinets in common sizes.  Recently I helped remodel the staff kitchen in a house owned by my local Boy Scout council.  We were able to do a complete kitchen remodel using Home Depot kitchen cabinets and premade countertops for less than $2000.  And this was a fairly large kitchen.  There were 8-foot and 10-foot walls of cabinets.  My teenaged son and I completed the entire project in two days. 
The other quality option, as you mentioned, is ready-to-assemble cabinets.  I've had good success with cabinets made by IKEA. I used IKEA cabinets for a kitchen remodel in our office, and found them to be very well-made and durable.
So either way, you can create an affordable kitchen remodel.  If you're still on the fence, here's something to think about: Ikea has a lot more design options than Home Depot, and also gives you more custom interior options like pull-out drawers. 
Good luck, and show us before and after photos when you're done!