Which devices benefit more from the stillpoints? cd-players, power amplidiers, loudspeakers, tube-, solidstate?
CD players or transports yes! - because they do a great job of draining off micro-vibrations from mechanical devices. Amps (even tube amps) I think not so much. Ditto turntables. I have mine under my Wadia transport, and incredible as the TEAC VRDS mechanism is, the Stillpoints enabled some very bad discs to track properly -- and that's the only evidence I have because I couldn't hear any improvements in sonics (doesn't mean there weren't any ;-)
I have found the Stillpoints to work well under my ARC CD3 MKII. They produced the widest and clearest sound stage compared to an assortment of other approaches. The shrill sometimes found in piano recordings in the second octave above middle C were noticably smoothed. I have purchased another set to try under my TERES TT.
The Stillpoints are very effective. I use them under the aluminum shelf that my Galibier sits on. Probably a bit more effective than a sandbox and much easier to use. I've not used them anywhere else in my system so I can't say what else they may or may not benefit.

For my tube amp and the power supplies of my preamp and phonostage I prefer Series +2 rollerblocks to drain vibrations off of the transformers. These are another very effective vibration control product. The results I hear are increased micro-details and air around the instruments.
I'll second Stillpoints with risers beneath transports and cd players.
I have stillpoints products under all my components. I don't know what your budget is, but stillpoints cones and risers in comination with their component stands worked wonders under my CD player and preamp. I also found the component stands to very effective under my speakers. The cones and risers by themselves definitely produce benefits, but the combination of cones and risers with component stands was highly synergistic.
I purchased a trio of Stillpoints from Music Direct for my VAC Renaissance preamp. This is the best tweak I have ever used! Most tweaks result in subtle differences and many are certainly not an improvement. The clarity and definition in the bass is staggering. The Stillpoints do exactly what they claim.

The VAC is a substantial preamp that is built from 6mm thick aluminum plates and posts. It is solidly bolted together and has an outboard power supply that sits on a separate spiked Mapleshade platform. The preamp is sited on a deluxe Justarack whick has 1.5 inch thick MDF shelves. The rack is positioned fairly close to the right speaker so it is subject to a bit more sound energy.

I have been working on different room acoustic treatments lately and the Stillpoints have made an absolute remarkable difference in the sound of my system.

I've switched back to my N802's and look forward to trying the ESP's once again after new carpet arrives in a couple of weeks.
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I have got a set of stillpoints risers and inverse risers.
Do you have any suggestions especially about the use of the IRs?
Apopira, are you referring to the mini-risers, or the full size ones? And what are you using the SP's for/under? I'm very knowledgable about using Stillpoints as well as the Stillpoints Component Stands.
Had Stilpoints under CD player ,AMP etc Then used SYMPOSIUM ROLERBLOCKS SERIES 2 ROLLERBLOCKS JR and SE under AMP . ROLLERBLOCKS much better bass definition sounstage etc. MUCH MUCH Better!!!ROLLERBLOCK JRs with TUNSTEN CARBIDE #10 BALLS cheaper as well!!
Ebm (and others ;--)
So much depends on how the Stillpoints are actually implemented/installed, meaning:

Pointed up or down?
Using what accessories?
Are the accessories fastened or just "sitting there"?
Are they in a configuration of three or four?

I find too many people just "throw a set of Stillpoints" under something (because that's about all you can do with most other devices) and expect miracles. Stillpoints can indeed do miracles, but the user has to do the work of discovering the best way to install/configure them for a given application.

I have a friend in the Tucson Symphony Orchestra who uses them under his amp and power conditioners and, on the Component Stands, under his speakers and individually between the low and high frequency sections of the speakers. (Dat's a lotta Stillpoints!) AND (with all that!) claims he can hear the difference between pointing them up or down!!!

Who am I to argue?
Apopira2, I have no recent experience with the individual StillPoints, but I am shocked with the impact of a StillPoints component stand under my amp. I am also going to use such component stands under my new speakers.