You can only work with a single catalog at a time. (As a refresher, Lightroom images are mapped in databases called catalogs, in which which you can create collections, and Lightroom only supports a single open catalog.) However, the app will attempt to sync with whatever open catalog you're logged into and will offer to switch to that catalog. So, for instance, if you're working with the images from a machine at the office, then go home and log on with a different machine, you can switch to syncing with that catalog. To do so wipes the previously synced catalog from your device. It can also import photos from your camera roll and automatically sync them back to the desktop.
This architecture is a mixed blessing. On the plus side, it keeps your iPad from storage overload. However, it also means that unless you're working serially on different projects, it's constantly wiping and resyncing. Whether that matters to you depends upon your workflow.
The interface is generally well designed, and as long as you're familiar with desktop Lightroom you'll grasp it pretty quickly, as it maintains the same sensibility and logic without violating any iOS interface conventions. There are essentially five working views: a grid view, filmstrip view, adjustments, presets and crop. But some capabilities aren't immediately findable. For instance, it wasn't clear to me at first that tapping on an aspect ratio flips it or that selecting outside the crop box lets you rotate. But overall it's pretty straightforward. There's also a quick reference of all the relevant gestures, though there's only eight.
In adjustment view, selecting a numerically determined operation pops up a slider. That's fine, but I like to enter values, which is faster and more precise when you know what you want. With presets, you get thumbnail previews, which is nice. However, the preset list frequently appears right over the center of the image, which is pretty silly.
Adjustments and presets mimic the built-in options available in LR desktop's Quick Develop panel; it doesn't support custom saved presets, which to me is a big negative. If you use presets in your workflow, you create custom versions. As a highly imperfect workaround it does allow you to copy the previously used settings to the current image.
An icon within LR desktop indicates which Collections and images are set to sync, but it really needs some indicator for images that have been edited within the mobile app as well as a way to filter them. I want some way in desktop LR to be able to view only the mobile-edited versions so that I can cycle through them, tweak adjustments, and generally make sure the changes are correct, since the iPad's screen gamut and dynamic range can't match that of a good desktop display. It may be possible to kludge this using the rating support added in version 1.1, though the app still lacks color flagging, another annoyance.
I did get it to work somewhat seamlessly in conjunction with Photosmith, which has far more powerful organization tools -- it allows you to edit metadata, color code and rate images, as well as give you a higher-resolution view for better ability to judge sharpness. Even an edge-detection view (like manual-focus peaking in a camera) would help in the absence of extra pixels, especially on lower-resolution devices. Version 1.1 added the ability to create and sync back a custom sort order.
Though there's still quite a bit missing from the app, there's enough in Lightroom mobile to help desktop users who need remote editing or viewing.