I went to Berlin for the first time in September 2006.
Museum Island was the first place I had scheduled to visit.
Between the Altes Museum, German National Art Gallery, the Pergammon Museum and Berlin Cathedral (I attended Gottesdienst at 10 a.m. Sunday morning), I must have spent 22 solid days there before I even looked at the Brandenburg Gate or the Reichstag. All were wonderful to visit: the Pergammon Museum is a real "jaw-dropper", and the Berlin Cathedral is Magnificent. I heard the organ playing and a choir from Rostock, with a lovely soprano soloist, sang beautifully.
The "Dom" appears "Catholic" in decoration but, when you see the statues of Luther, Zwingli, Melanchthon and Calvin, you know it's Lutheran. The Cathedral Crypt, where several generations of the Hohenzollerns lie entombed in their ornate funeral monuments, in well worth a visit.
My only disappointment was that the Bodes Museum and the Neues Museum were both closed for renovation. (Hopefully, I'll see them next timed I visit Berlin.) But, just on the west side of the river, lies the wonderful Museum of German History - not actually on the island, but right "next door".
The only thing which spoils the island, is that ugly, half- demolished monstrosity, which used to be called the Palace of the Republic - but who needs to waste time looking at that?