I normally tend to value an actor that's capable of performing a role; versus someone that can't act for beans, but looks perfectly like the character. Take Heath Ledger for instance, most people didn't think he could pull the joker character in "The Dark Knight." Let alone top Jack Nicholson that played him previously before that. And, look how that turned out. He not only nailed the character, but he became the character himself.
Or if you want to take this further, we can go back to when Michael Keaton was cast to play Batman back in the 80's. Nobody back then thought he could pull off batman. Hell, there were even petitions from fans to get Batman recast, as many people did not like the idea of the guy who played "Mr. Mom" to be Batman, and this was all before the internet even got started. Now, people to this day still argue that Keaton was the best Batman; even though I prefer Bale myself. However, it's a personal preference; much like how everyone has their own preference on who they think the best James Bond was.
As for the the casting of Paul Giamatti, I can see him pulling off the role, as he is a great character actor in ilk of how Heath Ledger was. However, a lot of it really depends on how the character is written, and it's obvious they'll need to use CGI to make him look bigger as the Rhino obviously. Although I think Jason Statham would be a better choice for Rhino, but that's just me.
As for Jamie Foxx playing Electro, I have no problems with that as long as he can nail down the character, then I could care less if he looks exactly like the original character. As Heath and Keaton proved, it's all about the delivery of the character more so than the looks. Take Shaquille O'Neal for instance, he looked EXACTLY like John Henry Irons aka Steel from the comics. However, since Shaq is such a mediocre actor, he not only botched up the role, but it was laughably pathetic to watch. Would you really want an actor that looks EXACTLY like the character, but can't act for beans?
Or would you rather have a character actor that may not look like the character exactly, but you know they would totally immerse themselves into that role? Granted, it would be nice to have someone with both qualities, but if you could only choose one, then i think the choice should be very obvious.