This latest version of Grammarian Pro has been rock-solid for me (Leopard and my G5 iMac), as was the version before this. Although I did not have some of the stability problems some reviewers in 2007 had with earlier versions, I agree that the program was in need of some bug fixes and updates, and the advent of Leopard seems to have spurred some much needed development and improvements.
As a professional writer and editor, I really love this program and highly recommend it. I've used it for years now and consider it part of my professional "toolbox." It's easy to use and easy to set up, and I especially like how much I can customize Grammarian. You can customize system-wide or program specific hot keys for virtually every function and you can customize the level and type of the grammar and spelling checking (Academic, business, casual, grade level, etc.) to an amazing degree.
The grammar checker, spell checker, thesaurus and dictionary are *far* superior to those in MS Word. Best of all, Grammarian Pro is available system-wide, and the level and kind of grammar checking and spell checking can be customized for each program you use, if you want to -- one kind of checking in MS Word or Scrivener, for example, and another when using a text editor to write code or HTML and another when using it in a web browser, or PIM, or spreadsheet. The level, kind, and amount of checking are up to you.
Of course, there's no substitute for the writer knowing and learning the basic rules of grammar, and getting an "ear" for what good writing sounds like. And GP doesn't catch every mistake, and sometimes it flags non-problems or even makes dubious grammar suggestions. But on the whole I find I'm far less likely to make bonehead grammar mistakes (not to mention spelling errors) when GP has my back, so to speak. GP can be a good learning tool, too. For example, if you're prone to using too much passive voice in your writing, you can set up GP to have it nag you every time it comes across it, and this can be a great way to improve one's writing.
I hope this doesn't sound like an advertisement. I have no relationship with this software or company other than as an end-user. But I do speak as someone who has used this program for years. Grammarian Pro can be a great help to anyone who writes. Version 1.8.1 is a solid update, and I'm happy to give it a 5-star rating based on my daily use of the program.