AFAIK,LinFu 是建立在 Mono.Cecil 上的。
I would say that PostSharp.Core has more high-level features than other frameworks, so it's less difficult to use for larger works. You can work at low level too, but not at binary level (by design). You could do an assembly merger/shrinker using PostSharp, but the fact that it compiles back using ILASM would set some limitations (for instance that even internal members should be named). On the other side, having ILASM as the backend makes it much easier to develop on the top of PostSharp, since ILASM verify many rules and you can easily read the produced MSIL code. PostSharp even let you put comments into MSIL to help debugging code generation.
Another point: if you want to do a custom aspect (for instance, you develop a database engine and want to deliver a persistence aspect), you need much more than just an MSIL rewriter. You need an AO infrastructure that will do much of the job for you. When you are developing a custom aspect, I would say 1% of the work is specific to your custom aspect, 39% is the aspect infrastructure, and 60% is the MSIL rewriter stuff. I am often able to program a very specific aspect in a couple of hours based on PostSharp.
So, back to your question, and of course with my own biais, I would say: if you want to write an obfuscator, merger/shrinker and so on, rather go for Mono.Cecil or Microsoft.CCI for the only reason that their license if more friendly than PostSharp's one. But if just want to develop a custom aspect, using PostSharp will make you save weeks, and you would be surprised by commercial conditions we could offer if you plan to redistribute PostSharp.