ISEESAW  There are some reviews here that are missing the point of this product.

If FileMerge and/or commandline tools will meet your needs then, by all means, use them and save yourself a few bucks. But if you need some more advanced features, Guiffy is a good, professional product. In my case, I bought it for the graphical three-way merge.

Sure, it's implemented in Java and looks it. But this is a tool for getting work done, not sitting back and admiring the Core Animation transitions. When you're drowning in 12 variations on the same codebase (what were they smoking?), as I was, the java-y controls won't bother you at all. You'll be too estatic that what seemed like a difficult, error-prone, weeks-long process of updates and verification is actually going to be a short and simple task. There's nothing like using the right tool for the problem, and Guiffy is that tool when FileMerge, et. al. aren't enough.

(Note: I used the previous version, but I did not experience any problems with an earlier update, so I'm not too worried about 9.)  
(Version 9.0 (270))

MACTECHHEAD  A Java based tool. How sad when Apple even offers free developer tools on every OS X Installer disc.

Too slow

Too cumbersome

Too unstable

And what a cluttered ugly GUI

Even if the developer paid me the $59 I still wouldn't try this rubbish again.  
(Version 9.0 (270))

FINBAR  I have been using Guiffy since Version 7.2 on my G5 Power Mac and just installed 8.2 on my new MacBook Pro running OS 10.5 (Leopard) and it is working there too.

I particularly like the inline difference highlighting of the file compare. When we got a project folder update from our outsourced contractor with Unicode file and folder names and some case differences in the names --- Guiffy made it clear what was going on. I wish all our other tools had such useful features.  
(Version 8.2)

AYMAN ABU-MOSTAFA  Guiffy is the best GUI application for file differencing and merging I tried. I tested it on MacOS 9, MacOS X and Windows. I also used it in three different ways:

1. As a stand-alone.

2. As a plug-in to version control applications; StarTeam and CVS.

3. As an API library for adding file differencing and merging capabilities to existing applications.

The author has been very responsive to my suggestions which often found their way to subsequent releases.  
(Version 6.1 (B 116))

ANONYMOUS  Ridiculous price for a bloated application. Can do the same with a couple of unix tools and the free FileMerge. The terminal is less user friendly but it works. This app is unstable.  
(Version 6.1 (B 116))

ANONYMOUS  Unmarked installer that doesn't inform you about what it's doing ... then it crashes. Be careful...  
(Version 5.4 (B 103))

MATT O'BRIEN  PROS: The compare and merge functions are well done. I especially like the character-by-character highlighting of differences.

CONS: It's a java application so it can be a bit slower than native software on some platforms.

CONCLUSION: Overall a good product. And the company is very responsive to comments from its users.  
(Version 5.0)

KEEPIN ITREAL  Wow. Imagine a poorly done version of FileMerge that you have to pay close to $100 for (and that's a *user* license, by the way; they want more money if more than one person uses the app on the same computer). The cross platform nature means they can torture a vast audience with this shoddy software.  
(Version 5.0)