I've been lurking these boards for the past couple months and wanted to drop a note of thanks to everyone here. You're all very supportive!
Also, as a benefit to others, a little about my experience...
I took the Kaplan GMAT classroom prep course ($1300!!) and found it worthwhile but probably not worth the money. I think that people who are reasonably motivated and can stay with a good study schedule can avoid Kaplan.
Here are the results of my practice tests:
Kaplan Diagnostic (linear) - 680 (43/39) [taken before I had done any studying]
Kaplan CAT 1 - 630 (39/37)
Kaplan CAT 3 - 640 (41/36)
Kaplan CAT 4 - 650 (50/33)
Kaplan Test A (linear) - 750 (55/41)
Kaplan Test E (linear) - 740 (56/37)
PowerPrep Test 1 - 770 (50/44)
PowerPrep Test 2 - 740 (51/39)
Actual GMAT - 760 (51/42)
As many other people have found, the Kaplan CATs are much harder than the real thing. The quant questions in those exams really challenged me mathematically, helping me know the difference between when I really knew the answer and when I was pseudo-guessing. The verbal questions, however, didn't help as much...Kaplan's conception of hard verbal questions was just weird. I really didn't agree with many of their explanations.
Good luck to everyone in their GMAT journey!