Willian had a really strong finish to the season and looks like a decent transfer for Arsenal. It's a free signing and when he wants to be, he's a good player. In this Arsenal team he will probably be a starter.
It's a free signing, and to me that's only what makes it a good deal for Arsenal, I do not quite understand Arsenal's transfer business, first they gave David Luiz another year contract and now signing Willian who's above 30, and past his peak. Arteta needs a good squad to compete next season, but with their transfer business thus far, I think next season we'll still see the same Arsenal we have become used to.Regarding the Thiago deal - I really hope it's true. Liverpol probably waited and tried to get Wijnaldum deal done, since he's not signing this would be a perfect transfer. A superior player to Gini and an improvement to an already great team. Happy days 
The midfield is the only area I think Liverpool need one or two new players. Milner is getting old and Henderson is getting injured often. Thiago is a nice buy, and will fix some midfield issues Liverpool has, the price tag is also very affordable and imo it's a good deal.