I've been using Google Chrome ever since it was first released by Google but I don't see it replacing Firefox anytime soon. While the speed is arguably faster, it lacks some strong features to make it worthwhile using. Don't get me wrong, I like the slick UI and all, and for that reason, it is best used for occasional browsing that are somewhat non-functional. I define functional browsing such as bookmarking, blogging, web development testing and debugging, interactive web games, etc. It's simple enough in a sense that it gives you just enough for your typical web browsing needs, but not enough for more advanced browsing. Whenever I use Google Chrome, I always run on scenarios where I say to myself, "Oh yea, this is not Firefox. I wish Chrome can do that too".
I'll point out some scenarios where I feel like Google Chrome is sub par in comparison to Firefox. I know that Chrome is in its early stages unlike Firefox which has been around (version 3 as we speak), and it will only get better with time. Since Firefox is known to be an extensible browser, the add-ons that were created for it were meant not just for the sake of being "cool" but as well as to increase productivity in duration of our web browsing experience. After trying out what Google Chrome has to offer, I have to say that the add-ons feature is what makes Firefox a complete browser but that's not to say that Chrome has its own advantages also. I like the fact that each browsing instance is independent from each other, which means that a crash would only bring down that instance whenever a crash occurs. Among other things such as an entry page with "most visited sites", that's about it.
In Firefox, I have an add-on that allows me to synchronizes all my bookmarks and saved passwords in multiple computers, so if I need to bookmark something in one computer, I know that can access that page at a later time from another computer. Another add-on is FireBug which allows me to debug a web page and find out under the hood, what's going on behind the scenes after the page has been rendered into the browser. I can also post to my blog using ScribeFire, another add-on that allows me to post a blog within Firefox itself without leaving the browsing screen. I have more, but this is what I call functional browsing because I can do more than just your typical click here and there type browsing. You don't want to get carried away with too many add-ons though as this will only weigh down your Firefox since it can be resource extensive. I just try to keep it as minimal as possible, with just the functional add-ons that I feel are necessary that I regularly use.
So in conclusion, I like Google Chrome but it lacks the add-ons functionality that Firefox possesses to make it an all-in-one browser solution. At least for now. I'm pretty sure though that Google is already working on something that would at least compete with Firefox, at least in the realm of extensibility. I've heard some past rumors that Google released Chrome too early in able to capture the web 2.0 crowd but can't remember the exact details. It lacks a major feature that I love about Firefox, which is the add-ons. Once Google includes such a similar feature, then the competition is open.