Right! Here's my review after day 1 with the Underworld. For reference, I drive 275'-300', and max out at 325' or so. Not a big arm by any means, but I'm learning!
What I threw and how long I threw it
I have a 172 VIP and a 168 Tournament. I spent a lot of time with the VIP today: played a round, and also spent a good bit of time throwing it in the field. I picked up the Tourney from a local disc seller towards the end of my afternoon out there, so I didn't spend nearly as much time with it, but I did throw it enough to feel I got a good sense of how it compares to the VIP.
172 VIP
I love the VIP; it's exactly what I hoped it would be. It's a notch more understable than even the mid-160s GL Rivers I've thrown, but it's not what I would call flippy by any means. Thrown smooth with just a bit of hyzer, it flips flat, actually turning less than I expected, and finishes with a tiny fade. It responds to annys amazingly well: just a tiny anny angle gets it turning, with a small fight back to the left at the end that's enough to keep it from rolling but doesn't undo the left-to-right flight. It holds deeper hyzers amazingly well. I think the disc I've thrown that it compares to the best is a DX Leopard, but I prefer the feel of the Underworld more, and the Underworld is way glidier.
I have nose angle issues, and this puppy is definitely sensitive to those, but not colossally so. It will be good for my form. When I did throw it "perfectly", it flew straight for days. Definitely glidier than a River for me, and I got what were my longest drives ever with this disc. I'm totally sold.
It was moderately windy this afternoon, and the VIP handled itself better than I expected, even holding an anny without faltering while flying directly into the wind at one point. This disc is versatile enough to handle all of my stable to understable duties if I want it to: definitely wouldn't need anything between it and a Banshee/Eagle/Teebird/Stag/etc, in my eyes, although I do admittedly tend towards keeping things simple.
168 Tournament
The Tourney is definitely a different animal. It's the first driver I've ever thrown that truly seems "flippy" to me out of the box, and it's a lot more finicky than the VIP. I can see why those who have thrown Tourneys have been turned off, and I would urge them to try a heavier VIP before they make up their minds about the mold.
I'm still going to spend some more time with the Tourney to see if it can't be dialed in, but my initial instinct is that it won't really be worth the amount of effort that would seem to require. Onemoremile described it initially as a butcher rather than a surgeon earlier in this thread, and I had a similar feeling. Not sure if I need a disc this understable when the VIP seems perfect.
TL;DR
The lighter Tourney is much more finicky than the heavier VIP. The heavier VIP is a notch more US than Rivers, glides a bit better, and has comparable nose angle sensitivity. Heavier VIP is preeminently shapeable, turns on request, and even handles a light wind well. If you're worried about this thing being too flippy, go with a 170+ VIP.
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It was well worth the wait! And in the bag. Expect to see a few Rivers up for trade in the MP soon. Way to go, Westside.