This season is probably one of the better to have come out in a while; it is definitely above that bad season where Lana was a witch. When Clark escapes from The Phantom Zone, he inadvertently releases several prisoners and spends about half of the season tracking them down and taking them down. The rest of the season focuses on a new billionaire that's in town, Oliver Queen.
So what's so important about this guy, besides the fact that he is a rival for Lex? Well, how about the fact that he is also a rival for Clark. It turns out the head of Queen Industries is a modern-day Robin Hood who gets dubbed The Green Arrow (funnily enough by Lois, The Namer of Heroes). Just like most superhero first encounters, the two characters clash until they realize they are on the same side and ultimately join forces. What is interesting about The Green Arrow is that he has no superpowers to speak of. Basically, he is really good with a bow and he has some trick arrows, but his impact on Clark is great because he is the one that starts to pull our future Man of Steel out of the Smallville/Metropolis mindset and gets him to consider protecting the world as a whole.
The peak of this story arch is in the episode "Justice" when Queen gathers some of the other DC heroes we have seen from past seasons, Aquaman, Impulse (AKA Flash) and Cyborg, and forms a young Justice League. The whole team then goes after one of Lex's experiments. This is, by far, one of the most enjoyable episodes from the season and should definitely be savored.
Then, there is always the soap opera aspect that comes with Smallville. Lana and Lex's relationship gets closer as she moves into the mansion and they eventually plan for a wedding. Chloe meets up with her one-time fling briefly mentioned a few seasons back, Jimmy Olson. Lois and Oliver even get together for about half the season (but we all knew how that would turn out, didn't we).
I don't want to talk about the season finale for obvious reasons (it's quite a doozy), but I'll just say that Clark's hunt for the Zoners (those that escaped the Phantom Zone) goes up to the last episode.
Smallville: Season 6 comes with a whole slew of special features. The first two discs have a majority of them (but unless you've already seen the season, you might want to wait until after you've watched it before you view them all), the only exception being the deleted scenes that are spread across all of the discs. These unaired scenes come from the episodes: "Sneeze," "Wither," "Arrow," "Fallout," "Hydro," "Labyrinth," "Crimson," "Freak," "Combat," "Progeny," "Nemesis" and "Phantom."
The other features include the two "Smallville Legends" promotional shows that ran alongside this season, The Oliver Queen Chronicles and Justice & Doom. The Oliver Queen Chronicles were six web-based CG shorts that gave viewers the origins of The Green Arrow. When you first see these, you might be shocked at the lack of graphical quality in these shows, but mind you they aren't really meant to be viewed on your huge HD TV. These were designed to be downloaded to your mobile device, so don't pay too much attention to the visual quality and just take in the story. As for the other show, Justice & Doom are a series of animated comic books following the some of the adventures of the newly formed superhero group. Unfortunately, this often felt like a way to work in some blatant Toyota Yaris product placement.
The other major feature is an overview of The Green Arrow since his inception in the 40's. This is a nice insight into the origin of this character and what his role has been, not only on the JLA, but in the DC universe in general. It's definitely worth a watching for any DC and/or Smallville fans.
So should you get Smallville: Season 6? Obviously, if you already have the first five seasons, then this new one is probably already on your wish list. If you are trying to decide if you want to get into the series, then this is not really the place to start. It finishes off a cliffhanger and furthers several themes and plots that have been under development for years, and while you shouldn't have a problem getting caught up, you won't get nearly the enjoyment out of it that followers of the series will.