I don't have any first-hand experience with them, but if you're talking about the "spacers" that you insert between coils, you're right: they are a bad idea.

I've replaced my wagon springs with load carrying springs from FCP Groton. They did a good job of leveling the car, but they aren't very good at carrying loads (ironically).

I've researched two options -- both of which are available for our cars:
1) air assist shocks (from Monroe)
2) "air bag" spring inserts (from Air Lift)

I've decided to go with the Air Lift inserts. I've had them on a Toyota van I once owned and they worked splendidly. Plus I'm concerned about the sacrifice in ride quality of the Monroe shocks vs my Bilstein Touring that are on there now.

Here's a link to an Air Lift vendor:
http://www.accessconnect.com/mercury_mitsubishi_nissan_oldsmo.htm

My research indicated that the spings for a Toyota Previa are the same size as for a RWD Volvo Wagon... but I'd want to confirm that with the manufacturer.
In any event, $70 seems like a fair price for safer/more level load carrying & towing, in my humble opinion.

If I ever get around to doing this, I'll post my results.

Jeff Pierce
--
'93 945 Turbo (201K miles), '93 945 (140K miles/I maintain for a family member), '93 945 Turbo (sold w 145K miles), '92 Mercedes 190E (174K miles), '85 Jeep CJ-7 w/ Fisher plow (225K miles), '53 Willys Overland w/ Fisher plow (sold to a loving home)