Pokewalker

I've done a little research on what the Pokewalker can do, expanding the HeartGold/SoulSilver games.

Basically, you can transfer a Pokemon over from the main screen of your HG/SS game, and then pull it back over later, bringing back with it whatever you found in the Pokewalker (Pokemon, items, and possibly Watts?).

Feature 1: Gain experience and happiness
It's maxed out at one level for the Pokemon in there. 1 step = 1 experience. Not that great for low-level Pokemon, but tolerable (free xp!) for higher level Pokemon. The Pokemon will also gain happiness, which can help some evolutions (and find more Watts).

Feature 2: Gain items and Watts
Each course has its own items that can be found as you walk. There's a chance as you walk around that your Pokemon will pick up an item, sometimes Watts. The happier your Pokemon is, the more Watts it will find. (Steps also figure into Watts.)

Feature 3: Find wild Pokemon with the Pokerader (costs Watts, may need some steps)
This is a simplified battle system of Attack/Dodge/Catch, similar to Safari Zone found in previous games. There's only 6 different Pokemon per course, but you can only find 3 different ones each time you load a Pokemon into your Pokewalker. It'll take you a certain amount of steps to have some Pokemon show up - reduced if you have a preferred Pokemon in your Pokewalker - every route has 3 Pokemon-types that just work better than others. Also, if the Pokemon you bring into the Pokewalker matches any of those types, it gives you a greater chance to find rarer Pokemon.

Feature 4: Item Dowser (costs Watts)
You can spend Watts on item-finding - check the moving grass, get some items. Pretty light activity. (You're also limited to how many items you can carry.)

Feature 5: Connect Pokewalkers
You can have your Pokewalker connect to a friend's Pokewalker, allowing you to trade items. When you connect, you'll also send a copy of your team in your HG/SS game, so your friend can battle against your current team (and vice versa). You'll only be able to connect with each Pokewalker once in a day.

Watts do reset when you transfer out, but I don't know the specifics...they might even reset daily. Certain courses will become unlocked as you gain more Watts, which suggests that you can transfer Watts into your HG/SS game. I thought I read that you can connect Pokewalkers to trade courses with friends, but couldn't find any of that information.