I would suggest ypu to have look at the One Step Checkout by FmeExtensions. All extensions are backed by free lifetime support, free upgrades and 30 days money back gaurantee. Following is the Magento connect link of the extension with 41 reviews.
The choice of such an extension depends on your requirements. For example, you may need a responsive checkout - this also needs to be taken into account.
One more iportant thing is vendor quality, make sure you get best value for your mioey, and buy from those who are ready to give back your money within a particular period if you need.
You can consider our Single Step Checkout.
I would also recommend you to consider a multi-step checkout like this one http://www.magentocommerce.com/magento-connect/gpmd-checkout-suite.html. We have not tried it yet because everybody is still crazy about the one step checkout layout, but user experience studies in fact show that people have some trouble getting around the big form that one step checkout shows them at once - they tend to think that fields in all three columns are related to each other, continuous updates with ajax are confusing, etc. Instead, when there are several steps the user has to fill one form at a time and is guided through the process with a progress bar.
Here's an article about different checkout assessments if you are interested http://baymard.com/blog/checkout-process-should-be-linear. Sorry for so many letters
Magecom | Magento e-Commerce Development
We have developed "One Step Checkout" extension with magnificient features. Please review it from here.
There are many kinds of this extension which are pro but expensive. This One Step Checkout extension is pro one but the prices is good for developers like us.
I would like to suggest you our Enhanced One Page CheckOut extension. You can have a look at our extension at http://www.softprodigy.com/store/enhanced-one-page-checkout.html
I highly recommend Magecheckout to your store on account of their high quality and a reasonable price. It has various features that I will show you below:
1. Easy to customize
Easily configure one step checkout module to suit with your store design. You can chose 1, 2 or 3 columns style and custom the color that depends on needs of yourself.
Your customers will be suggested automatically their addresses without delay by Google Address suggestion and Post Code anywhere. It gives customers have most convenience and make purchase fast.
3. Promotion display
Checkout promo is a smart and intelligent marketing tool arms you encourage customers buy more with up-sell advertisement promo on checkout process.
For example: Get an Extra 10% Off with Order of $100 or More, Get X free when buy Y items, $10 Off Your Purchase Of more than $100.
4. Cross-sell Products
Cross-selling is one of the special features which makes customer spend more money whenever they fall in love with other suggested products. These ones that come from other categories which are somehow related, will be shown at the bottom the checkout page.
There are many features that you can see at Magecheckout website, hope this helpful for you.
Magento’s checkout was almost 2 times better, but I’ve seen cases where it’s a completely different situation. You can’t just take a case from some other store and assume it will bring the same results for you.