Pickford - Looks like he'll be a reliable keeper for a long time if we hold on to him. Could be great with the right coaching and more experience. Worth it.
Klaassen - He could be a good player but not for a defensive team that attacks so directly. I'm not sure why we thought he'd be a fit for Koeman's style. Do your homework, Everton!
Oneykuru - Cheap and looked great for Anderlecht this season. There's nothing we could have done about the injury. Hopefully he returns to full fitness and gets a work permit for next season.

but I really liked the thinking behind this move.
Keane - I'm not entirely sure what to make of him. He defends well when we sit in our own penalty area and I think he's a useful passer. On the other hand, he's slow and looks awful when asked to defend in space (it's possible that this has something to do with the cut on his foot not healing properly but who knows). And I question the logic of signing another low-block defender, especially after Koeman spent an entire year talking about pressing.
Sandro - Another example of our transfer team not doing their homework. Goal tallies don't tell the full story for attacking players. This guy scored a bunch from distance and odd angles last season and even given regular opportunities here, he was unlikely to produce like that again. It was a bargain fee but the wages were crazy for what he really was.
Rooney - Awkward fit in the constantly changing, disorganized mess that was Koeman's Everton. Has been an integral to all of our competent performances in a deeper midfield role. Long-term, we need a better midfield passer but for now, I'm made up that we have him.
Martina - Was a complete mess under Koeman. Settled down under Allardyce and generally defended well in an unnatural position.

for how he's handled himself and how much he's improved.

at the three year contract and the decision not to sign a better player (or players).
Sigurdsson - Has a touch of class and works incredibly hard without the ball. Not nearly involved enough in build-ups or our general attacking play though. He's a good player but with a definite ceiling which is a problem for how much time and money we invested in this deal.
Vlasic - I've enjoyed watching him on the occasions he's played (good strength and balance, looks like he thinks before he tries stuff) but who knows if he's actually any good.
Walcott - If we want to target experienced players, this is how it's done. His underlying numbers (like shots, expected goals, expected assists) looked good throughout his Arsenal career and he was no longer wanted there. He's been our best player since he arrived. Needs to sustain this level for more than half a season but
Tosun - I wasn't the biggest fan of the deal given the league he came from (Turkey, where 30+ year old former stars regularly top the scoring charts) and the price. I'm still not sure what to make of him. Could be a genuinely good player, could be a "meh" forward riding a conversion hot streak.

for now, with the potential for a

if he keeps finding good shots and scoring.