One nice thing about a stem raiser (if that's what you're talking about) is that once it's installed, you can then switch stems without having to worry about readjusting the headset - the stem raiser has its own bolts, just like a stem, so it holds the headset in place. That's just a nice little convenience to have, if you're experimenting with stems.
The downside of a stem raiser is that it will tend to raise your bars quite a bit, so there's kind of a gap between the top position on the old steerer, and the bottom position on the raiser (unless you get a stem with a different rise, or turn it over, I guess).
All in all, I think stem raisers are fine, I've never had any problems with them. Some don't like the aesthetics of it, but otherwise they are very good for dialing in your cockpit fit if you need to get the bars up some. Especially with bikes that have had their steerer tubes prematurely cut too short.