I drove a bfriends trix with these. It feels and sounds like the engine is bolted directly onto the firewall. The sound resonates into the cabin. But you also get that performance boost where there is no, or little loss in shifting and power to the wheels. They are also only for manual trannies.
Well worth it, in my humble opinion. The front takes about 20 minutes. THe rear mount can take 2 hrs if you have to kind of figure it out as you go. There are a few how-to's at another site. PM me for the link.
Helps with wheel hop.
Better power transfer.
Some say better shifting due to lack of tranny torque.
Produces a resonance around 4k rpm which can get annoying.
Steering wheel vibrates. This lessens when the mounts settle in. You also get used to it.
Using the AC can produce a stall like feeling when the compressor kicks on.