I have replaced PDAnet with MyWi because they seem to conflict if used together.
In my case I wanted to upgrade my 3G to the 3GS but was not motivated till I heard about MyWi. Yes, PDAnet worked fine but I wasn't happy with the general speed of my iPhone 3G. Upgrade for me was $400, but since I figured I could sell my old iPhone for $200 I would break even as far as buying a new 3GS with an early contract penalty. MyWi seemed like an incredible innovation so I took the gamble, downloaded the app onto my Macbook Pro desktop while my iphone 3G PDAnet was still active (I do not have any other internet access source at my home) and left for the Apple store, MBP in hand. While in the store I mulled around, still sitting on the fence, doing some research on one of their new Macbook Pros on display to decide which was better for me, the Incase or the Mophie Juice Pack air, both able to double battery time on my new 3GS, if I decided to go for it.
Finally I bought the 3GS, sat down with it in the store, connecting it with my own MBP laptop. Mind you, I did not jailbreak this brand new iPhone; I just installed the Rock your phone software on it from the desktop app. Installation was so easy a caveman could do it. Before I knew it I was looking at two new icons on my 3GS; a ROCK app and a MyWi app., all installed in the store using Apple's own hotspot! I then purchased the mophie Juice pack over the incase, not that I think one is better than the other; they both seemed to have good reviews.
I later noticed in wifi mode MyWi would drain the battery, as expected. But WAIT. When I finally got around to using the mophie there was no more battery drain! The 3GS actually charged fully while using MyWi in wifi mode, and I do not mean only by the mophie, i mean by it being plugged into the wall or laptop.
I cannot guarantee this will work for you; it may be just a wierd anomally in my case, but it has never failed to charge while in wifi mode. Apparently the mophie did something, not sure what; now it recharges in wifi mode completely, without being in the mophie case. Was it really the mophie case or the fact that I did not jailbreak my 3GS that caused it to recharge in wireless mode. I am not perfectly sure, but if anyone decides to use this software without jailbreaking try to recharge in wireless mode before buying a $79 mophie, just to be make sure.
Wifi mode can be iffy and I find it seems to depend on how strong your 3G signal is. Also I notice my old Powerbook 17" will not share the wifi signal as long as I have a WEP key enabled. Otherwise my friends seem to be able to use the hotspot MyWi creates with no problem, with or without a WEP key. USB tethering is FAST, faster than PDAnet, though it may be helped by the fact I am on a 3gs now and not an iPhone 3g.
I am posting this pic using MyWi in wireless mode, from my MBP in OSX Leopard, since for some reason I cannot upload a pic to Photobucket in Bootcamp Windows XP.
Otherwise USB or Wifi tethering is excellent in Windows XP; I can surf anywhere since MyWi does not use Proxy.
One other bug on my system; sometimes MyWi will remain on until you turn your phone completely off and on again by holding down the on-off button.
Despite these few bugs MyWi is great for me and well worth the one time fee of $10. I can't guarantee it will work as well for you, but this is in my opinion about an unbiased review as I can honestly make. I say it is just awesome that I can create my own hot spot with just a touch of a button, and it is an incredible app that replaces routers costing 20 times as much.
. The idea behind it is to be able to manage all of your independant iPhone software with one rockid. This helps greatly when you get a new/replace your iPhone - you don't need to contact each independant software vendor to transfer your license. We also have an enhanced version of mobilesubstrate (Rock Extensions), which has additional error handling to keep your phone safe. 
