Hi SMS offers a nice skin for your texts, but it doesn't have many of the features you probably need from iPhone's app. It's a decent way to transition from it, though.
As you might have guessed, this app is designed to look exactly like iPhone's text-messaging inbox. There is one thing keeping it from doing that exactly--the ads. They're nowhere near as bad as some apps, but they wouldn't appear in an iPhone app. They tend to get in the way if you have a smaller phone, too. Since Hi SMS is only a skin for your messages, you won't have any of the features you'd expect. That means your iPhone's beloved Emoji won't show up for the party. In fact, the app doesn't have any advanced tweaks to speak of. The settings menu just leads you to a place to rate and download more of the developer's apps.
If you don't mind sacrificing features for style, this app is an okay way to move away from the iPhone. Hi SMS is a dead ringer for the messaging app on iPhones, at least as far as looks, if not features.