My one sentence evaluation:
If you MUST use Internet Exploder then Gliffy is your solution, otherwise download Chrome (or other html5 capable browser) and use use Lucid Charts.
The long Version:
As a long time Visio user, it used to be one of two microsoft tools I missed when working on linux/Mac. Beyond not really working on linux, it's just too damn expensive for the amount of time I spent using it. I'm an occasional user who needs to slap something together that is only slightly more professional than a napkin (although I've used napkins on occasion) or ascii art for illustrating network diagrams or software architectural components.
In the last 5 years or so, a couple of online tools have emerged that let you do this inside your web browser. I've used and been a paying customer of both Gliffy and Lucidcharts and will say they are both pretty good tools and if you're someone like me who like me, they can meet your needs.
Lucidcharts is a better put together product from a useability perspective, has more widgets and seems to be innovating much faster. For example, Lucidcharts has iphone templates, works on iPad, and can import visio. This last one is a killer feature that Gliffy has failed to implement for almost 4 years.. Lots of excuses, but no feature. Gliffy has abandoned their customers by not implementing this and regardless of how difficult it might have been to implement, personally, I think this feature alone will stand to propel lucid charts ahead of gliffy.
Gliffy uses flash and therefore will work on old browsers and IE, but you're likely out in the cold trying to use Gliffy on an ipad. All told, I'm very satisfied with Lucid charts and highly recommend it. Gliffy isn't a horrible product and I'm sure might find a niche in large corporations that don't use Visio already and are stuck using IE, but it just feels klunky in my hands and hasn't seemed to keep pace with what I need.
I just noticed this post five best online diagramming tools and neither Gliffy nor Lucid Charts are listed...not sure why, maybe they aren't in the five best?