I definitely recommend getting the photopass and have as many pics taken by the photographers as possible - even without characters. When I was there is April, they had a Value Pack available in the photo store for $59 I got a
Disneyland Photo Gallery CD and a Disney Photopass Photo CD (voucher so I could order the photocd after I got home and edited all the pictures). It was definitely worth it.
We saw most of the characters randomly. As previously mentioned there are the set meet and greets: Princess Fantasy Faire, Tangles, Pixie Hollow, Merida, Toy Story (in CA). We also saw many characters in ToonTown during the ToonTown early entry pass that came with our tickets. We saw: Mickey, Minnie, Donald, Goofy, Chip & Dale. We also saw many princesses near the castle. We made it a part of our day to walk past the castle often to see if we could catch anyone there. Some of the characters will have photopass photographers and some won't, so have your camera ready too. In CA it seemed that Mickey and Minnie were there at times and Goofy was near Goofy's Sky School.
Even if a photopass photographer is near, they cannot take pictures of Mary Poppins and Bert. I was told that Julie Andrews owns the copyright to them and doesn't allow it.
During our trip we saw:
Aladdin (Goofy's Kitchen)
Alice in Wonderland (Town Square)
Ariel (Near Castle)
Buzz Lightyear (Changed Characters just as we got there so no signature)
Chip & Dale (saw them many times throughout both parks)
Cinderella (Near Castle)
Perla (Near Castle - she is one of the mice with Cinderella)
Donald Duck (Toon Town)
Eeyore (Critter Country)
Evil Queen (Near Castle)
Fairy Godmother (Near Castle)
Gepetto (Town Square)
Goofy (Goofy's Kitchen, Toon Town, Goofy's Sky School, Town Square)
Jesse (Toy Story)
Mad Hatter (Town Square)
Mary Poppins & Bert (Town Square and Near Castle)
Mickey Mouse (Toon Town, Town Square and in CA, but I don't remember the location)
Minnie Mouse (Goofy's Kitchen, Toon Town, Town Square, near the Airplane in CA)
Mulan (Goofy's Kitchen, PFF)
Pinnocchio (Town Square)
Pluto (Goofy's Kitchen)
Princess Aurora (Near Castle, PFF)
Rapunzel (Tangled)
Rosetta (Pixie Hollow)
Snow White (PFF, Near Castle)
Tigger (Critter Country)
Tinkerbell (Pixie Hollow)
White Rabbit (Town Square but he left right before we got his signature)
Winnie the Pooh (Critter Country)