I just wrote this piece below for our agency blog. I'd categorise Twitter List in teh Really Useful technology camp. Like the Remeber the Milk service. A lot of tech onnovations or services don't add much to your work or home life effectivness, I think twitter Lists do...

== post written for Agency Blog ==

The way businesses and individuals use twitter could all change with the introduction recently of Lists. Lists are a way to classify, differentiate and also explore the twitter ecosystem - really quickly and effectively.

Tweeters come in all shapes and sizes, hues, creeds and interests and they talk with a huge spectrum of voices. So it can be a bit confusing trying to follow and engage with them all in your mixed twitter stream.   For example, jumping from a friend’s social update to a marketing ‘guru’s’ latest insight to a gig date announcement always felt a bit schizophrenic to me. Also, I selectively add interesting people to my follows when I see who else my contacts are connected to -  but it can be a slow process.
And ‘Follow Friday’s’ (http://mashable.com/2009/03/06/twitter-followfriday/ ) is great to see recommendations but is limited in the number of ‘experts’ or plain favourites someone else recommends in one tweet.
That’s where Lists come in. In a couple of minutes you’ll start to differentiate your different tweet streams and also start to spot some very interesting list your contacts have curated that you can sign up to follow).
When you create your own lists, it’s interesting to note how you classify your followers.  Curate specific experts in a field or businesses by one industry type or recommendations of great Writers and so on .. Lists can be used in lots of different ways.
Mashable suggest the top 10 uses here http://mashable.com/2009/11/04/twitter-lists-uses/
 
Personally I have lists to differentiate Digital Marketing folk, people who make me chuckle (I don’t actually LOL when reading stuff), photographers I can learn from and Tech or outright Geek  info sources..
For example I haven’t got time to trawl through lots of followers’ tweets on the chance I’ll get a Digital Tech update, so I just select that specific List and immediately I’m more up to date than I would be through reading a magazine or news website.

And how others classify you is also interesting. I’m variously classified as a Tortured Dad, as a Marketing Bod, Photographer and just ‘Digital’.
Although none (sadly) of these lists are titled ‘Masters of the Digital Universe’, lists will definitely be used to gauge someone’s business / social kudos and ‘value’.
 
Two interesting aspects of lists :
(1) you don’t have to follow someone to see their tweets .. if you subscribe to someone else’s List of (say) Politicians, you can see all of their thoughts but not actually follow any of them directly.
And (2) you can have a Private List (as in only you see it), so you could add people to your hidden list that you want to keep an eye on but not directly follow (therefore avoid alerting them to the fact you are following them).  In essence Twitter Stalking (is that a ‘Tweeting Tom’. )

There’s still some limitations with lists and no doubt the functionality and uses will change so I won’t detail all of those here,  and as always Robert Scoble puts forward a great overview:
http://scobleizer.posterous.com/twitter-lists-limitations-bugs-impact-and-bri

Get list building and list collecting and let us know what you find!