Having used Lion and Snow Leopard side by side since a few weeks after Lion released, I can honestly say Lion is a step down from Snow Leopard in terms of reliability, usability, and resource efficiency. That said, you may not notice these shortcomings depending on how you use your Mac. I'll summarize the basic reasons for my assertions here for the sake of clarity:
Reliability
Every application that comes with Snow Leopard is stable and relatively bug-free. Lion, on the other hand, continues to plague a number of users with wireless connectivity issues and has the buggiest version of iCal that has ever been released. I have not personally experienced the wireless connectivity issue, but all the Macs at the office running Lion had to have BusyCal installed to be able to connect to our CalDAV server calendar, whereas the ones running Snow Leopard had no issue connecting via iCal.
Usability
Lion adds a lot of fluff, much of which is optional, and I am honestly fine with that - I'm not one of those blowhards that screams about Launchpad daring to exist even when I don't have to use it (and in fact I kind of like it). Mission Control, which replaces Exposé and Spaces, however, is an unmitigated disaster.
Have several windows open in a few applications and want to see them all at once (say, Photoshop and InDesign)? Sorry, you can't do that in Lion because Mission Control chooses to immediately obscure any window with other windows from the same application with its window-grouping mess. Apple even tried to soften the blow here by allowing you to use the four-finger-spread gesture to slightly spread out the windows - guess what, it still leaves parts of the windows obscured.
Want to drag that window from one space to another? Whereas you used to be able to very intuitively just click and drag while in Spaces view, now you have to first switch to the space where the window resides and then drag it to its destination.
Want to fullscreen a movie to your external monitor while you work on something light on your laptop display? Nope, Lion took that away because with Apple's fabulous new fullscreen implementation, all monitors you might be working on aside from the one with the content is grayed out.
Mission Control is a horrible implementation of what I'm sure were well-intentioned ideas that ends up being objectively less functional than what it replaced - as far as I can recall, this is the first OS X release to actually take away features. If Steve Jobs were alive, I imagine he'd give the team behind it a speech similar to the infamous one he gave the MobileMe team. Lion took a huge step backwards in multitasking workflows and window management.
Resource Efficiency
I'll be blunt here: Google any Benchmark comparison between Lion and Snow Leopard, and you'll see that Lion is either identical or a minor downgrade. This is the first version of OS X to ever achieve this. But given that benchmarks often don't reflect real-life usage, let's focus on that.
Lion takes longer to both start up and shut down than Snow Leopard, thanks to the only mildly useful (and not very graceful unless you have an SSD) feature that restores your session upon startup.
Lion uses a ridiculous amount of RAM compared to Snow Leopard - my coworker only uses her Mac mini for office tasks (spreadsheet, word processor, calendar, email) and the base 2GB of RAM wasn't enough to keep it from lagging. Let this sink in. Lion is so inefficient with RAM that many of the machines Apple is shipping it with are incapable of running it smoothly even when doing the least intensive of computer tasks. Sure, many people have more than 2GB of RAM anyway, but consider where this places your machine now. Suddenly 4GB is the amount of RAM you need for light multitasking and 8 GB is a "good" amount of RAM. Your performance is going to get shot in the foot with this upgrade unless you have 8GB or more of RAM. And all for a few shiny new features that mostly add more flash than substance or actually make the user experience worse.
The one and only thing I can see someone upgrading to Lion for is the iCloud functionality that Apple refuses to put into Snow Leopard, assuming you own iOS devices.
All in all, I strongly recommend against upgrading to Lion. Apple doesn't need any more dollars encouraging them to go in this direction with the OS. I can only hope 10.8 brings back what Lion took away.