I like it.
It hasn't become my primary way to access twitter, but it does do some things very well.
Before I go too far, I must stick in the required disclaimer. Although, I work for Intel, I tweet only my own opinions. Nothing I say should be construed as a recommendation on Intel's behalf.
One of the things that works best about it, is that it runs in your browser. That wouldn't seem like an advantage, but it actually is. The reason why that works well, is following a link becomes very easy if you used a tabbed browser, simply click on the link and the text pops up in a new tab very fast. Much faster than from a standalone twitter app. Moreover, when you close the new tab, you are back on twitter.
Another thing that pluggio does really well, is how it handles your inbound and outbound messages. In particular, it has a special category that holds just the tweets you send out. You might not think that is useful, but it really is. If you are trying to figure out if a tweet got sent or not in times of twitter problems, just look at the tweets you have posted and if it is there, you sent it. They also do the same thing with in-bound and out-bound DMs, so you can see just the ones you've received or just the ones you've sent. I really like both of those features and for those alone, I will keep using it.
However, it's main claim to fame is reading RSS feeds and helping you get that content into twitter. It is very easy to follow RSS feeds with pluggio. More importantly, pluggio helps you turn those articles into tweets. If you are trying to build a following by offering links to interesting content, this can be a big boon.
The second feature it offers is scheduling tweets. It has a couple of ways of doing this. A schedule at a specific time option, and a rolling tweet option. If I used pluggio as my primary interface to twitter, I would probably use the rolling tweet option as my primary way of RTing people, as it helps you avoid overloading your stream by sending too many tweets at the same time and then having dead air later.
One feature, pluggio offers that is in several other clients is the ability to manage twitter, facebook, and other social media accounts from a single client. If you want to put your message out across several sites at once, this might be the solution you need.
Now, it is time for some balance. There are two things I get from my primary interface to twitter that I don't get from pluggio.
The first one in multiple visible columns. I have my tweetdeck open all the time, with about 8 columns visible. That really helps me keep up with the "way too many" people I follow. Pluggio only shows 1 column at a time.
The second one is "tweet shrinking". Since most of what I do is RTing other tweeps, I often have to shorten their tweets. Again, there is a tweetdeck feature that gives me a head start on that and pluggio doesn't have that.
The last thing I should mention is that pluggio is "nagware". The free version is truly no cost, but it does gently remind you after a while that to support pluggio's development some people need to get commercial copies. It's reminders are gentle though and not too intrusive.
In the end, you will need to try pluggio before deciding whether it is something you will like or not. I did and I do.