A few days after Amazon released its Kindle for iPhone app, it became the top book-related download, nudging out the Stanza and Fictionwise eReader applications. And while it’s a decent application that’ll likely attract some defectors from the competition, it has its flaws.

First, the good: the iPhone app gives you access to all of the books you’ve purchased at the Kindle store. It also syncs to the furthest page read in an e-book, so, in theory, firing up the iPhone app will take you to the exact spot where you left off reading in your Kindle. When it works, it’s pretty slick. But it doesn’t always, and the annoyed user then has to manually thumb through pages to find where they left off. Also, strangely, there’s no search functionality.

A major limitation is that the app will only give users access to the books they’ve purchased. If you subscribe to any magazines or newspapers, you’re out of luck; those are only accessible on the Kindle. Amazon also really dropped the ball in one big usability aspect: you can’t buy an e-book from within the application. The “Get Books” button takes users to a screen that advises you to visit amazon.com’s kindle store.

Stanza, previously the most popular e-book reader for iPhone, has more features: you can read newspapers and magazines, buy titles from within the application, download thousands of free e-books (the vast majority are public domain, the rest are publisher promos), and search e-book text. But Stanza is just an iPhone app, and while the iPhone screen is fine to read for the duration of a subway ride, nobody’s lining up to read War and Peace on their iPhone.

Amazon’s promotion of the iPhone app as a complement to the Kindle is spot on. It isn’t the most feature-packed reader and has irritating limitations, but it loads quickly and displays text as sharply as you’re going to find on a small LCD screen. Its kinship with the Kindle will make it the go-to ereader app for Kindle users, while its extensive catalog of e-books—nearly a quarter million—is the largest available and will certainly attract users.