I've been using Outpost for a bit, and this week I had a chance to try out Base Tent [App Store link]. While not as pretty as some of the others, it works very well.
When first launched, Base Tent asks for your login criteria and begins its initial sync, which was much quicker than I anticipated (note that I did all testing over EDGE). Four tabs label your Dashboard, Projects, To Dos and Milestones, and they use the same color coding you'll find on Basecamp.
Below is a series of icons that let you re-sync, access the app's help pages or add and remove accounts (I only have one account, but I imagine those with more than one will appreciate being able to jump back and forth). Best of all is the option to find tasks, etc. assigned to a certain contact. Of course, the purpose of dropping ten bucks on an app like Base Tent is ubiquitous access to Basecamp.
Click any item in the Dashboard to jump directly to it. Reading and responding to messages is intuitive, though longer threads take a bit of time to load. The app includes an option to view a message title, post date and author or the full text, which is nice. Responding to a comment is just as easy as you'd expect and syncs as you publish.
The Projects page lists all aspects of a given project. What's really nice here is the option to move between projects and contacts with a click, just like Basecamp's "Switch to a different project" feature. I had a very easy time adding to dos, responding to messages and setting up appointments from the road.
Base Tent syncs upon launch to look for changes. Unfortunately, this process commandeers the display until it's finished. Additionally, and I know this is a bit superficial, but the Dashboard screen looks a bit cluttered. I'd like for the "Completed By" text and date to be a bit smaller. Finally, I know they're going for the Basecamp "look" with the tabs, but they seem cramped in the iPhone's display.
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The good news is that none of my complaints are about the app's performance. Interacting with Basecamp via Base Tent, which is the point after all, is pleasant and reliable. Many of Basecamp's customers, myself included, depend on the data in Basecamp for our living.
The developers have told us that a major update of Base Tent is complete and currently awaiting approval at the App Store. At $10.00 US it isn't the cheapest solution in the App Store (nor its it the most expensive), but it's worth it.