EBay on iPhone is notable for what it doesn't include just as much as what it includes. It's a free, very small utility that lets you monitor your current auctions as a seller and bid on auctions as a buyer, among several other frequent tasks. It also has a simple auction search function. But a few key features are missing.
What it can't do is start a new auction, check out of a completed auction that you won, leave or view feedback, or look at your eBay messages. These are pretty crucial features for any serious eBayer, and their omission means you'll end up at eBay's page back at your PC anyway.
I found one annoying bug. EBay on iPhone censors the high bidder's name when you look at item details. It's supposed to happen only when you're looking at items that aren't yours, but it happens even when you're the seller.
The best use of eBay on iPhone is to keep an eye on bids for stuff you're selling, or to quickly bid on an item when away from the computer. It would be nice if eBay were to add more advanced functionality to a future version, or even release a full-featured pay app. For an eBay addict, it would definitely be worth the money.