The application “free for a limited time,” and considering how the App Store works there’s a reasonable chance that it’ll remain free forever to anyone who download now (there certainly are workarounds if the developers really want to charge you later, but I don’t think it’s quite their style), so you should probably at the very least download it now, just in case, even if you’re not sure you’ll want it.
Getting a web browser on my iPhone that can auto-fill passwords is so tempting it might convince me to buy the desktop application (I had a free license for a while, but every new browser release requires a 1Password update, and I definitely miss it now that all the browsers on my Mac are incompatible with my version), but I’m still resisting because I don’t like the idea of having to use an add-on to have auto-fill when it should be standard functionality, and more importantly I’d really rather not have to use 1Password’s integrated browser instead of Mobile Safari (no matter how Safari-based it is, obviously). And, yeah, obviously that’s also Apple’s fault.
Meanwhile, the iPhone application’s interface for entering new passwords on the phone itself is pretty much unusable (you have to type the URL and the field names, instead of just saving the data from the password prompt as you do on the desktop), so the application is just about useless if you don’t have the latest version on your Mac in order to synchronize your passwords over wifi (with secure encryption, according to the description). But 1Password is an excellent program that I recommend wholeheartedly, and encrypted over-the-air sync sounds very good.
P.S. Oh, I forgot: it can also store password-protected, encrypted notes. So, you see, there’s a reason to download it even if you’re not a 1Password user. (If you need to secure notes on your iPhone, I think 1Password is a good choice of an application to trust.)