It basically depends on what kind of games you play, and whether you play single player or multiplayer. The short answer is that you don't need a second Move or a NaviCon for one person, but they can help augment your experience in a specific subset of games.
The second Move is only usable in two games shown so far: Sports Champions (for bow/arrow in archery, sword/shield in gladiator, and I think beach volleyball) and The Fight (out in October). Both can be played with one, though you'll need a regular controller in one hand for The Fight, but both are better with two. If you're interested in multiplayer, the second Move can also serve that function, which is nice. Sports Champions comes in a bundle with the Eye and a Move controller for $100, which is a pretty good deal, and it's one of the best Move-only launch games.
The NaviCon is used for certain games to move your character around with the analog stick. So certain genres use it, eg. FPS, TPS, Adventure, etc. RTS games like R.U.S.E. will most likely be compatible with it too, in order to have more hotkeys. However, the left side of a regular controller can substitute for the NaviCon. You can try putting it in your lap and using the left stick normally, but without your right hand to see if it's comfortable. If it's not, and you are interested in playing many of the games in those genres, then get a NaviCon. You definitely won't lose any functionality if you use a Dualshock in one hand though.
I made up a list of games that are confirmed for use with the NaviCon on this board. It's not totally inclusive, but it could give you an idea of what kinds of games use it. A full list of games that have been announced for Move support in general is in the Starter Bundle forum, or in the next two links.
So here's a question about the gladiator game then. If you don't use a second Move controller for the shield, do you just use the regular controller, and it uses the gyroscope instead? Do you think it'll have much impact on the accuracy/gameplay? Because I'm getting this with the intention of using it for multi-player; buying a second move controller for that is already expensive, getting 4 of them so I can battle with someone else seems over the top.
Not that I've tried it, but they say that Gladiator lets you switch between controlling the shield and sword by pressing one of the big buttons. You don't use anything in the other hand if you're not using two Moves. I'm pretty sure that you can't block and attack at the same time anyway, so it shouldn't affect gameplay. I'd say it's less immersive (maybe less intuitive?), but looking at videos of 2 people play with one controller each, they don't seem to have much of a problem.
^^ not use the Playstation Eye? I don't think the Move controllers work without the Eye. One can buy the Eye without any Move controllers and play certain games and videoconference (free worldwide) etc, but I don't think there's any use to getting a Move controller yet no Playstation Eye. More specifically I'm not sure how a Move controller would track one's head and send the signal to GT5...
Thanks for the post Locke! I've looked over both..
OT-sorta: I cancelled my pre-order for the Move & Nav controllers & Sports Champ. (even though I own a PS3 Eye-I had picked them seperatly because so) because besides a few titles, I'm not that impressed yet w/the list (per my taste in game types) -yet-. I think Sony should [still could] have made at least one more "pack", with just both controllers. When Sorcery comes out, that from the list that uses both controllers, would be a good incentive for example. As of right now the NAV controller is lowered under gold box & yesterday the Move wand was. If they [Amazon] puts them both on sale, I'll pick them up. I just don't see spending $75 aprox. right now for controllers I just wouldn't use alot at the moment.
I bought it and love it so far. I only bought the bundle. Sports championship makes you hold the move button to control the shield if you only have one controller. This works fine but I can see how 2 would be better. I love the gladiatoer game. As for the move joystick i didnt buy it and have enjoyed playing mag with the DS and the move. Its cool. I play better since i didnt like mags classic controlls much anyway. If you get it at kamrt you get $25.00 future game credit with it. Perfect for adding the analog controller for just 4 bucks.
I boutght the starter kit and a second Move controller and already regret not having bought 3 extra move controllers. The games that use two per person are some of the best and kind of scuk without the second controller per person. I have found no need for the navigation controller yet and frankly I think games that would use it might be sensory overload on how many things to keep track of, at least for my family. There are so many buttons and ways to control things by discrete movements on the main controllers that its more than enough for us...