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Pros
Pro Debugger and inspector
CL programming paradigm is to "teach" your program what to do by defining functions, variables and classes (this way it is similar to Smalltalk). As a benefit, the great debugger comes for free: the behavior of a function can be tested, debugged and altered on the fly, no need to start the program or recompile the whole program. Debugger also comes with the inspector that lets one to see and alter the components of objects and variables.
Overall, debugging options include: tracing, debugging on error, restarts, inspecting variables or results of expressions, including altering their contents, macro-expansion debugging.
Pro Lots of production-ready libraries available
Common Lisp is arguably the most useful Lisp to "get things done". One of the main reasons of this is the large community behind it and the large number of libraries and frameworks already built to solve virtually any problem you may have when writing programs to be used by actual users.
Pro Image based runtime
The state of the program may be saved and reloaded as an image, supporting safer modification of the running program. New code may be compiled into the image as the program runs, while late binding ensures that symbol redefinitions take effect throughout the program.
Pro Almost as fast as, or faster than C
Some compilers such as SBCL can be faster than C or other low-level languages, and most compilers can generate fast native code.
Cons
Con Variable capture problem
“Macros are vulnerable to a problem called variable capture. Variable capture occurs when macroexpansion causes a name clash: when some symbol ends up referring to a variable from another context. Inadvertent variable capture can cause extremely subtle bugs.”
Con Too many implementations
There are a lot of Common Lisp implementations out there (CLisp, SBCL, CCL, just to name a few). Some implementations are free and open source, others are proprietary and cost quite a bit. Though none of them follow 100% the Common Lisp standard for example, SBCL fails about 50 official ANSI Common Lisp conformance tests. Having so many implementations may make it hard to develop new libraries because you would have to test it for several different implementations.
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Flagged Pros + Cons
Pro Almost as fast as, or faster than, C
Some compilers such as SBCL can be faster than C or other low-level languages, and most compilers can generate fast native code.