Normally the Lonely Planet guides for iPhone are $15.99, so any way you look at it, getting the San Francisco guide (iTunes link) for free is a great purchase. At worst you'll see if you like the Lonely Planet way of doing things in a travel guide and at best you might discover something new in San Francisco. Yep, the guide is free in anticipation of WWDC, allowing attendees to do a little sightseeing if Apple's sessions aren't enough for you.
I've only spent a bit of time with the app (I'm not in San Francisco, so testing the "nearby" functions won't happen until next week), but if you're familiar with the Lonely Planet guide books, you'll be pretty familiar with the content here. The app has a really long list for the table of contents, something I felt could be handled better. If you want to find travel information, for example, you'll have to scroll somewhere about 40% down -- this is not optimal if you need info in a hurry. In fact, scrolling through longs lists is so annoying Apple gave the iPod app a search tool and Coverflow browsing. Luckily the LP guide provides a text search which I found quite useful.
Browsing content is terrifically easy, with a slim design that allows you to move forward and back between articles, increase text size or go back up to whatever screen you were previously at, like a search page. Speaking of those searches and the content, a cursory search for Moscone brought up no precise info on the convention center. It is mentioned in a reference to Yerba Buena, but otherwise lacks details.
The map suffers from static information and a lack of one-way street info (kind of important in cities filled with them, but only if you drive). I much prefer the maps and interaction of the AAA Discounts app, complete with animation and cleaner graphics. There are complaints about missing hotels and whatnot, but I've never fully trusted any guidebook to find every hotel and restaurant. Plus, there's always Urbanspoon and a plethora of apps for discovering new places to eat around you.
Obviously if you are ever planning to travel to San Francisco it'd be hard to turn down a free app so full of info. I don't think it'll replace anything like your Maps app, or Urbanspoon, or Yelp-based apps, but that's not the point. As a guide full of history and photos, hotel and restaurant data, workable maps and tons of "getting around" data, the Lonely Planet San Francisco City Guide is incredibly handy.