My sister bought me a Kindle 3 with 3G just after I had donated my library to charity during the holidays, and I've been using it every day since. It's a great device for reading books on. I'm already half-way through Iain M. Banks first book in the Culture series: Consider Phlebas. The screen is a bit more reflective than I had thought it would be, given that it's not backlit and is using e-ink, but it's not a huge annoyance and is certainly much more comfortable to read on than a laptop screen.
Every time I get a new gadget I look up tips and tricks other people have found and adopt whatever I find useful. For my Kindle the 4 most useful things I've found are:
4 - Kinstant: The feature that makes the Kindle with 3G worth paying for over the wifi-only version is that the 3G data access is free. Free 3G makes the Kindle a great Hitchhiker's Guide, since you have free access to Google Maps, Wikipedia and all the knowledge available on the net, almost worldwide! I think every Kindle owner knows that you can search Wikipedia straight from the home screen, but I didn't know how good the experimental Webkit-based browser would be until I tried using Google Maps on it. Kinstant is a start page for the Kindle browser. It has links to the mobile versions of sites like gmail, facebook and twitter, as well as links to news, local weather and even the mobile site for Project Gutenberg so you can get a ton of free public domain books loaded on your Kindle. The most useful part is that it uses Google's mobile proxy to let you visit sites you specify in a stripped-down, bare-bones version that is better suited to reading on a content focused device like a Kindle.
3 - Kindlefeeder is a service that lets you get blogs and other RSS feeds onto your Kindle, automatically if you pay a fee and manually otherwise. I use the free version and simply click the "send to kindle" button every morning to get the 12 feeds free accounts are limited to wirelessly delivered. It's a great way to substitute for a newspaper and since I don't need more than 12 feeds or automatic delivery the free version works well for me. Just make sure your Kindle is connected to wifi and you've set your charge limit to $0.00 on Amazon's "Manage Your Kindle" page if you don't want to get hit with the $0.15 fee every time you get a wireless delivery.
2 - Instapaper is like Kindlefeeder but different in that it's not intended to deliver RSS feeds but whole webpages. It makes it easy to send pages I stumble on while browsing on my computer to my Kindle for later reading without having to plug it in and save the page manually. You just create an account, drag the bookmarklet to your browser bookmarks panel and then when you see a page you want sent to your Kindle you click the bookmarklet. Instapaper will send your clipped articles wirelessly to your Kindle every week or daily, depending on your settings. You can choose to send over 3G or wifi, I use wifi since it's free.
1 - Calibre is a cross-platform open source ebook management program. I run this on my Linux laptop to convert pdf and other files to the mobi format, which the Kindle can read natively without needing to use the awkward zoom function. I was familiar with Calibre already, since I used the Mobipocket Reader app on my Blackberry Bold to read books before I got my Kindle and had to convert to mobi format all the time. There are lots of features available, enough that this is probably the only piece of downloaded software you'll need for your Kindle if you're running Linux and don't have access to Amazon's Kindle for PC software. Mac and Windows users will want to use this alongside that program, it's just that useful.
Update 1/21/11 - I just found out that Calibre can be used to email a collection of books from your computer to your @kindle.com or @free.kindle.com email address, allowing you to transfer your computer documents to your Kindle without even plugging it in or manually emailing files!