Well...
If you do play multiplayer, it will be addictive untill the next installment comes out. I still enjoy ssbm to this day. Since you don't entend on doing that, online play and the one player mode should be enough to keep you interested for a while. You'll get to see all the famous ninty characters collaberating together to take down all of nintys famous antagonists!

Online play will also keep your interests for quite some time...but it will never replace talking to your friends, pointing out your wicked ko's, and telling others how unfair your death was!

I'd still reccomend this game, it would be a whole new experiance, and if you got friend codes from other members on this site to play online, it would be more interisting altogether.

IMO, it's easier to decide for yourself. So, here are most of the features included in Brawl:

39 playable characters of varrying fighting styles. Another 29 major characters appear in the form of Assist Trophies, where they will come out and fight for you.

Subspace Emissary. A 10 hour single-player adventure mode featuring all characters in the game interacting in an intertwined story. 2 player co-op in this mode. 5 difficulty levels. Includes an RPG like stat increase system where you attach stickers to your character's stat board to boost their abilities.

Over 300 musical scores composed by the best song writers in video game history (Nobuo Uematsu, Motoi Sakuraba, Akihiro Honda, etc.), and the ability to pick what songs play and how often in each stage. These include the original scores from games, remixes, compilations, and even Brawl original songs.

41 different stages including returning stages from Melee.

A stage builder letting you build infinite combinations of your own unique styles and ideas. Can be uploaded to Nintendo for a chance at Stage-of-the-day or sent to your friends over the internet.

Classic mode, a standard fighting game 1 match after another ascent to a final boss.

Event Mode, where the game sets you up in unique situations based on their own history and franchises for challenging matches such as Pokemon Battles. Co-op available here too.

All the types of Brawling: 2 minute KO Fest, Stock, Team, etc. Not to mention the Special Brawl effects (Giant, Metal, Spring, Super Speed, Super Slow, many more, and any combination of the above).

A total of well over 1,000 collectible trophies, stickers, and songs.

Demos of all the games the characters come from: Super Mario Bros, Earthbound, Fire Emblem, etc.

Many minor game modes: All-Star, Target Test, Home Run Contest, etc...

Screenshots that can be taken from any angle or character's perspective after pausing the game. These can be sent online to your friends.

100-Man Brawl, a challenge against hordes of enemies to see how many you can kill. Also available in co-op and online co-op.

Stage of the day, a stage builder stage submitted by people around the world will be sent daily to everyone in the world who chooses to download it.

Recordable matches, you can take recordings of your matches and watch them or send them to your friends over wi-fi, just like the screenshots.

Online multiplayer. Friend Code matches, random play matches, co-op 100 Man Bawl and co-op homerun contest.

"Well over 1,000 collectible trophies, stickers, and songs. "

You're kidding? There were 350 or so songs and collectibles in Melee, and I used a lot of time finding that.

Another 100 plus major characters appear in the form of Assist Trophies, where they will come out and fight for you

Really? Darn, that's a lot.

I dont really think I have much more to contribute that naz and xBrawlx have not said. Basically, there is a TON of Single Player features. You will definitely spend at LEAST 20-25 just on that stuff alone (Possibly more). And I promise you that once you delve into the Single Player modes, you will want to player other people. Chances are, you know other people getting the game. You could play online with them or in person will them. (You can even choose to go through most of the Single Player modes in Co-op with them!). Who knows!? Maybe you will learn to really love the series and start entering tournaments like me! =]

@ naznatips, actually there are "only" 27 Assist trophies.

Ah, yeah sorry about that this was an old list before I had that number.  Also edited the trophies, ATs, and stickers part to saying "A total of" well over 1,000.  Don't want anyone confused there hehe.

 Let me clarify.  

Melee had around...100ish songs that could not be customized to different stages.  Brawl has 287 songs that CAN be played in any stage you choose.

Also, there are not 100+ Assist Trophies in Brawl. (Assist Trophies are different than Trophies).  There are however a good 29 AT's in Brawl (essentially Poke Balls without the Pokemon). 

 If anyone else has more accurate info, correct me please.  As far as my extensive research goes, this is all very accurate though. 

EDIT:  I have found evidence for 29 AT's. 

Advance Wars Troops
Andross
Barbara the Bat
Devil
Dr. Wright
Duster
Excitebike
Gray Fox
Hammer Bros.
Helirin
Isaac
Jill
Jeff
Kat & Ana
Knuckle Joe
Lakitu
Little Mac
Lyn
Metroid
Mr. Resetti
Muddy Mole
Nintendog
Princess Rosalina
Samurai Goroh
Saki Amamiya
Shadow the Hedgehog
Stafy
Tingle
Waluigi

There's an Isaac AT? You have a shot of it anywhere?

Man, when you see it all laid out like this, it's astounding what they managed to fit on a game disc. 

I didnt like zelda or M & S very much ut i loved Melle and know i will love Brawl, i would definetaly reccomend it over any other game currently out even though i have never played it, i have got soo much enjoyment so far just watching matches etc so playing it must be heavenly lol.