Well, they're not perfect, but if you'll be sewing for others or your weight fluctuates (like my sisters does), the ones at Joanns suffice.
I had eleven bridesmaids, four junior bridesmaids, and four flower girls for my wedding. My mom and I made all the dresses, and most of the bridesmaids were far away in town or very out of town, and so either could NOT make fittings at all, or only very seldom. We had to buy three forms but got them on super duper sale and so it wasn't that expensive and for what we were able to make the otherwise $1100 coutour dresses ($75 per dress), the cost was worth it divided by 20 people (mom made my dress and I was working two jobs).
For the very out of town people I contacted 1) a friend in the area who I trust implicitly when it comes to sewing and measurements and they sent me to people who could take them properly in their area if they couldn't do it themselves. 2) the costume shop head in the local university so that they could do the measurements (they know how to do measurements!) for money.
Then we adjusted the dress forms accordingly. We didn't have any issues at all. They weren't, as I said, perfect, but they really did meet the need!
So, if it's for you and you stay the same size forever, do spend the money on the good one. It'll last forever! But...if you will conceivably use them for others as well, or if you change sizes/shape frequently, it's my vote that the adjustable ones can be useful, though not as high quality.
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