I also need to say something about how combat works when you're declaring actions before the initiative roll, because it seems there was some confusion on this point. Here's an example how it works in my games (grossly simplified, but you'll get the picture):
There are four players, a fighter, a magic-user, an elf, and a dwarf. They encounter 5 orcs and a troll. One of the orcs is dressed up in fancy gear and hanging towards the back, and the players know this means it's a shaman. It all kicks off.
DM: Okay, it's combat, what are you going to do?
Fighter: I'm going to charge the orcs and attack them.
Magic-User: I'm going to cast an acid arrow at the troll.
Elf: I'm going to fire arrows at the shaman. (Knowing that it's a shaman and will likely be casting a spell.)
Dwarf: I'm going to try to get in front of the Magic-User and protect him from attack. (Knowing the magic-user is likely to be preparing a spell and wanting to make sure it goes off.)
Meanwhile, I decide that the orc shaman will start casting magic missile at the magic-user, and everybody else will attack the elf, wanting to kill their hated race enemy first. Everybody rolls initiative.
The order comes out as follows, fastest first: fighter, elf, orcs, dwarf, troll, magic-user, orc shaman.
The fighter charges forward into the mass of orcs. The elf's view to the shaman is slightly blocked so the player asks the DM if he can move a little to get a better angle. Because we're grown-ups and we can negotiate and the rules explicitly state that the decision about what happens is down to the people at the table, I agree. The elf moves a little and shoots the orc shaman, ruining his concentration. The orcs pile onto the fighter. The orc shaman would have cast his magic missile at the magic-user at this point, but can't because his spell has fizzled thanks to the elf. The dwarf player asks if he can change his action to attacking the orcs instead - I rule that he can't, because it's too different to his initially-stated action (in real terms, he's already started moving to block off the route of attack to the magic-user, and would be too flat-footed to suddenly change his momentum). The troll charges past the melee towards the elf. Finally, the magic-user casts his acid arrow at the troll.
From this, you can see, hopefully, two things: the idea that you can stop spells before they are cast adds a whole new tactical dimension to combat; and just because you're declaring actions before initiative it doesn't necessarily mean things are appreciably slower, more difficult to resolve, or less flexible. (There may be cases where it is so, but not enough to detract from the benefits gained.)