I too just bought a set of JPX800 Pros, 4-AW, with DG S300 shafts (I almost bought Project x 5.0's (or 5.5's), but there was a $15 upcharge - another $100 bucks! *sniff*). I have played two rounds with them so far - both better than earlier rounds this season. I got the New Decade Grips. They are *awesome*. I've used Lamkin Permawraps and Wynn Excel's before these (plus whatever comes on Cleveland wedges and irons), and these are the best so far. Lamkin's were also very good, though, too. Wynns seemed to wear out quickly, and were a little "squishy" feeling. Probably should not have gotten the squishy Excel grips, then. But hey, it was college, I was experimenting...
I tested Burner 2.0, Callaway X-22's, and Ping G15's and the JPX-800 regular alongside the pros before I bought (outside at a range, probably 100 balls hit with each over the course of 2 weeks). I am a 15 handicap USGA.
Length:
The Burner 2.0's, X-22's and JPX-800 reg's were longer than the Pros. The 2.0's and X-22's were quite a bit longer - 7 iron was going 160+ and 6 iron would regularly reach 180, sometimes 185. Crazy long (for me, at least). On most courses, though, I don't need that much extra distance, so the extra length was not a big seller to me.
The Pro's for me are about 155 for a 7i and about 165-170 for a 6i. Twice I hit a 5 iron into a green 180 away, and twice I found my ball 10 yards past the hole - so the length is good on the pros (much better than my old set of Cleveland TA4's), just not crazy long.
Feel:
Pro's were the best. JPX regulars, Callaway and G15 were actually pretty good too. Burners felt a little "stiff" for me.
Accuracy:
They were all really good, in general. The 2.0's were a little harder to keep on line, but I suspect that was because the club length and lofts are a little higher than the other sets (i.e. I was comparing 7i accuracy, but in realy the 2.0 was a 6 iron-ish comparable, so was a little less accurate). The Pros gave me a little "less" on off center hits than the rest of the sets, but not penalizingly so (I still had a lob wedge into the green).
Overall:
They were all good clubs. I don't see how anyone can go wrong with any of the new irons out there. I had borrowed a set of MX-300's for 3 weeks prior to buying, and I think that familiarity gave the edge to the Mizunos. I have to say, the MX-300's are remarkably close to the 800-pros. Very close. If you can get those used, or at a discount, it might be worth it, but most pro shops won't fit the MX-300, since Mizuno won't customize an order for those anymore. Weak!
I was very close to getting the JPX-800 regular line, figuring I could use the extra forgiveness. Callaway X-22 (and Razr's) were also very close 2nd place.