Do isolation devices really help?

No they don't. Just think about this. After you put all your electronics on the best cones, pads, concrete, whatever; how do you keep them safe from airborne resonances? They can't. Case closed. You should couple, not decouple. I could go on, but is would be a lot easier for me, and you as well, if you (I just wrote this review, a week ago)go into the archives and look up Sistrum Mini Monitor Platform. There's oodles of stuff in that review to get your gray matter perking.

Yes, they really do help (or at least change the character of the sound you hear). Check out the following article accessible via the archives page of www.stereotimes.com : "Electro-Mechanical Convergence: How can it affect your audio gear?"

Isolation devices in conjunction with equi support (after all, the equip IS sitting on s/thing, isn't it?) produce the following general effects (VERY generally speaking):
*shift in tonal balance; i.e., it may shift the perceived reproduced spectrum, say, upwards...
*extend the spectrum; ideally both ends, all other *good* things remaining equal (i.e. more perceived bass, treble). Speakers are obviously correctly placed!
*change tonal balance, i.e. part of the spectrum is perceived as being more pronounced than before -- hopefully, correctly so.

In short, you're "tuning" or getting rid of the deleterious energy that seamingly compromises the performance of yr equip.
These are the product of coupling, decoupling, and generally protecting equip from vibrations of internal &/or external origin. Sometimes, rendering a piece of equip rigidly inert to movement also helps...

I.e. I'm just repeating the above in a different way! For specific products & practises & tricks, there's lots of literature in the archives.!

Yes they help. Warren is correct in that they do not have much effect and can actually work against air born motion, and in some cases coupling is the way to go here. As has already been mentioned different devices will have different effects on different equipment. Some subtle, some large, some will even make things sound worse. In general you have to experiment to figure out what type of devices work best for what pieces in your system.
Another generalization is that coupling speakers to the floor cleans up the soundstage, tightens the base. This is in line with what Warren has said. The speaker is creating air born motion and you would like it to remain rigid to create that accurately. Now, that said, there are those that use vibrapods or roller bearings under their speakers and swear by them. These devices have not worked for me for speakers, but I obviously haven't heard these other systems.
Other devices produce internal vibration. A CD transport is an obvious one and these frequently benefit by absorbing that mechanical energy with vibrapods. (again a generalization)
My amplifiers have huge transformers in them that have a 60Hz mechanical oscilation (all transformers do to some extent--but the really large ones are worse). I use a roller bearing system for these. It couples the transformer to the floor, but allows the 60Hz energy to be disappated laterally. This has been an extremely effective isolation device for me.
My examples show you three types of devices that work differently: absorbing energy such as vibrapods, coupling, such as spikes, and coupling with disappation, roller balls. Shelving can also make a difference, and I've been using a combination of granite and corian.
I do have a friend that has the ultimate isolation device. All the equipment is in a separate room from the speakers. Of course it's all isolated independently in the separate room.

Try Aurios under your CD player (I recommend the 1.2s). Then YOU answer the question.