A word of explanation - about Lisp power.
What I mean is not that Symbolics machines are the fastest CPU at processing (they are complete clunkers by today's G-Hz CPU hardware standards). Rather that they are superb environments for the programmer to use to get their brains around new designs.
We can all nip over to borrow the nearest PC word processor with its cooking Intel/AMD multi-cores inside if we need CPU power. But what help is there out there for the designer/developer, the software engineer who knows what they want to do but not how to do it yet, who is not painting by numbers - i.e. is working something out for the first time.
One often reads how Lisp programmers cost a lot. Of course untrue, but the point often missed in the reasoning is that these people are often doing very difficult things, which generally carries cost burdens. Think code mechanics v original thinkers (at least in that direction).
In summary, by power, I mean power tools for thinkers and designers. Not high
On lisp execution speed see Ken Anderson's insights, and perhaps Paul Graham's.
Perhaps the greatest value of working in Lisp, and even more of using a Lisp Machine, is that the environment extends the conceptual horizon for thinking people and designers. That is, ideas can be brought into life which otherwise might not grow. Of course the PC might be the platform on which to deliver the money maker, but first we need the idea, then comes the transition from abstract conception to functionality with which a user/customer can interact. As in reality, in the lispm world everything is interconnected, a freedom essential to allow the modeling in software of what we conceive. This is unlike the convention which is to work to the constraints that the technology determines. Plug&play can bring tunnel thinking, even if rapid visibility in a cloning sort of way. But someone has to pioneer the next generation of ideas for the big brands to have something to copy! Hope this doesn't come over as too cheeky/evangelical, and I'm probably mixing lispm issues with lisp itself (and anyway blowing counter to the winds of change).