Andy,
To answer your question, you need to divide VFP versions into two groups: 6.0 and below; and 7.0 and above.
Going from any version of the first group to any version of the second group is a must-have upgrade. You get lots of excellent producticity improvements - not least of which is Intellisense - plus many worthwhile enhancements to the language and object model. The upgrade will be worth every penny.
Going from any version of the second group to any other (later) version (e. 8.0 to 9.0) is less compelling. You've already got Intellisense and similar productivity features. If you do upgrade, you'll still get a lot of benefits, but whether it is worth the cost and effort really depends on what features are important to you, and how much time you can devote to learning them.
For me, 9.0 is far and away the best version yet, and was well woth the cost and time to upgrade. But your situation might be different.
I would add that if you are going to go to the trouble of upgrading (and it's really not that much trouble), you might as well go with the latest version.
Griff,
I certainly don't agree that 9.0 is flakey. On the contrary, I've had far fewer problems with it than with any other version.
What's your experience.
Mike
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Mike Lewis (Edinburgh, Scotland)
My Visual FoxPro site: www.ml-consult.co.uk