OK, I'm going to post something that is going to be pretty unpopular, at least judging by the other posts here.
After getting reqs and playing 80+ games on the suspect ladder, and despite where I thought I'd be voting when I began this suspect test, I am leaning toward voting to unban Victini.
When I think about how I'm going to vote on something like this, I like to ask myself three questions: does this pokemon overly restrict teambuilding? Does it invalidate any specific strategies, playstyles or 'mons? Is the tier more or less fun with Victini in it?
So, first, does Victini overly restrict teambuilding? Well, suspect ladders are hard to judge this off of, because people tend to way overprepare. That said, I think it's fair to say that every top team is going to have to account for Victini in their teambuilding, and probably have a couple of checks. As many have mentioned here before, Victini's unpredictability makes it dangerous (although I think that unpredictability is being overstated - thus far I've been able to more or less guess every 'tini set I've come across with nothing more than a damage calc and a look at what item they're using), so you can't just dedicate one teamslot to Victini and call it a day. On the other hand, many of the answers to Victini are actually really good, and exist across the spectrum for all playstyles. So, yeah, Victini definitely restricts teambuilding... but I'm not really sure it overly restricts it. When I look at some of the top teams from recent tournaments, most of them could be fairly easily adjusted to account for Victini in the tier.
Second, does Victini invalidate any specific playstyles, strategies or 'mons? Stall and offense are both doing just fine on the suspect ladder. Balance is a bit shaky, with the one-two punch of Mamo dropping and 'tini coming over to play, but I already see balance teams adjusting around the meta. As for specific 'mons... not really. Darmanitan is the only 'mon that I expect to completely drop out of high level play if Victini is allowed back into the tier - but let's be real, Darmanitan wasn't exactly a big presence in high level play to begin with. Entei usage will go down a bit, since a lot of teams can't really afford to stack two offensive Fire types, but Victini and Entei have different roles, so it won't disappear entirely. On the other side of the coin, there are a lot of already decent 'mons that get a bit of a bump in viability. Krookodile is stronger than ever, as are Hydreigon and Salamence. Mega-Ampharos, already a great pivot and one of the better answers to arguably the scariest 'mon in the tier (Mega-Pidgeot) now gets an additional major threat to check. Dragalge is only a soft check, but handles mixed sets superbly and OHKOs after rocks with Draco Meteor. Tyrantrum is finally getting a chance to shine. Fast offensive megas like Aero, Pidgeot and Beedrill are still great for their ability to outspeed and heavily damage 'tini. Basically, almost nothing that's currently a top choice has fallen from grace, and unlike the Serperior meta, Victini isn't forcing us to run a bunch of garbage 'mons like Sap Sipper Bouffalant.
Third, is the tier more or less fun with Victini in it? This one is tough because it's so subjective, but thus far it seems like Victini has made the tier more interesting, not less. I liked the UU tier prior to the Victini suspect, so it's not like I thought things were stale and need a change, but overall UU has felt just as much fun as always.
Basically, like Mamoswine, I think a lot of Victini's alleged brokenness exists more on paper than it does in play. Yes, theoretically it is impossible to switch into, especially if you don't know its set. On the other hand, its sets are pretty predictable. LO recoil means mixed or special set, no recoil but suspiciously low damage means Scarf or Ebelt lure (don't be a chump! check that damage calc!), and when it deals like 20% to something with U-turn, you know it's banded. It's ridiculously easy to force out thanks to its typing, average Speed and reliance on V-Create, and its typing means it can't reliably switch in on much either. Don't get me wrong - Victini is obviously a top 'mon in the tier, no question. I just don't think it's actually broken. Playing on the suspect ladder, the Victinis I came across weren't any scarier than, say, Mence or Slurpuff or Suicune - and honestly were less scary than Pidgeot or Feraligatr.
So, tl;dr version: Victini is obviously very good, but doesn't seem to be a negative presence on the tier. Unless something major changes between now and voting that sways my opinion, I plan to vote to unban Victini.