On the plus side, Roll20 provides an excellent experience during the sessions.  The Hangouts plugin has been very stable for me, and players on a variety of OSes and browsers have been able to tie in without any major issues.  The central uses for me have been map sharing and die rolling, and both operate smoothly.  Designating player tokens is relatively easy, and the players can then move their tokens around the map at will.  I have also paid in (as I prefer to support services I use frequently or heavily where I can), and as a result I can make use of the dynamic lighting engine for the maps.  While this costs a bit more setup time, it brings the players into the session much more completely by limiting their visibility to their direct line of sight.  I find it particularly good at ramping up the tension when the players are hunting down some evil creature they need to defeat but don't quite know where it is.

Roll20 also has its share of cons.  Biggest among them from my point of view is the setup time.  Building maps can be very time-consuming and nitpicky.  Despite the availability of a token and tile marketplace, I often find that common types of things (such as a lakeside map or a mountain pass) are not available and must be built up from smaller grid.  This is doable but time-consuming.  It is also clearly an area that Roll20's developers have somewhat outsourced to the community, and as such, I hope that it will pick up as Roll20 grows in popularity and capability.

Menuing is also sometimes problematic.  Popups often cover odd areas of the screen, scroll weirdly, or are broken up into tabs that don't quite make sense.  In most cases, this isn't really an issue; a little poking around close to where you think an option should live will generally bring you to it.  This is an issue that I think can be worked out with a little more attention to page and tab design; most of the key functionality is there, it's just not always made accessible in the cleanest of fashions.  Again, I think as Roll20 evolves, this will definitely improve.

I can't adequately review the wiki-like functionality or the character journals, as I keep all of that information on my private wiki.  Since Roll20 supports wikilike markup in character journals, I'm sure that a gaming group just starting up would be able to make good use of this for notetaking, though switching between tabs during a session might be tricky.  I tend to keep notes in a local notepad program anyway, only uploading them to my wiki after the session is over.  I would probably continue that were I to use Roll20 for my campaign notes.

Overall, I'm very pleased with the capabilities of Roll20 and don't think I'd have been able to run this campaign nearly as successfully without it.  I consider the paid subscription to be worth every penny if for nothing but the dynamic lighting engine, and I'll gladly maintain my subscription for as long as I'm participating in geographically-distributed campaigns.  I also can see that the team is obviously working hard to incrementally refine the webapp, something I very much appreciate.  I hope that the map-building engine will improve over time to make it easier for time-constrained GMs to build up encounters more quickly, and I hope that some of the minor bugs I've encountered are fixed as the tool is further developed.  Even with these minor annoyances, though, I highly recommend any GM who can't get all their players into one room to try out Roll20.

As a small postscript, this month's major upgrade will introduce character sheets to the Roll20 arsenal.  This introduction might lead me to move character data off my own wiki for the first time, as having those character sheets might really improve the experience for my players.  I'm very much looking forward to trying them out.