Visual Studio Online was recently renamed Visual Studio Team Services, which more accurately tells you what it is about. Sure, you can still browse and edit code, but it is just one feature, and not really a core one at that. On my current project I have had the chance to dive in a little deeper, and have a look at some of the features that VSTS has to offer. Although I have often been critical of these kinds of products, VSTS has been mostly a pleasent acquaintance.
Seen from 10.000 feet VSTS is devided into 5 areas: code, work, build, test and release. You don't have to use all 5, and they are nicely separated, so even for smaller projects you can gain what you need, without getting dragged in to an unneeded heavy process. For our projects we are mainly using the code, work and build parts at this point, and it has been fairly smooth sailing so far.
Code, is simply the core features that you are used to from TFS. So version control, that is easy to setup with Visual Studio and browsable from the web. I am personally a big fan of git and github, which I would still pick if it was up to me, but the experience has been more than fine. There are only the most basic features available in the web experience, but browsing history and manageing the code works really well. The big thing I keep missing is the nice distributed experience of git. The offline experience and merge resolution hasn't gotten any better, so for teams of more than a few people this will bite you over and over.
Work, is a clean and simple backlog, where you can assign tasks, specify acceptence criteria and comment as work progresses. It is not a big and fancy tool, but in my oppinion we pretty much never need big and fancy. To be honest, I have yet to see a project really gain something from having an "advanced" or complicated process. If you are in trouble, it is most likely the communication in the team that is bad, and no amount of rules and tools will fix that. So I have been happy using the work features, even though I am critical about the design, which doesn't really use the screen estate, so there is good room for improvement there.
Build, is that part of VSTS that I was looking mostly forward to using. With the new build model, it promises to be easily configurable and the visual editor gives you an easy to understand overview of the overall build process. Having used it for a while now, I think that it is doing a lot of things well, but some parts of the configuration is still error prone, and the support I received was very poor to be honest. We do however have a build process, for both serverside and clientside artifacts and tests, that everyone in the team understands - which I know a lot of teams that can't really claim for their processes.
So with this short review I will definately recommend that you to give VSTS a shot. I will surely look to use it going forward, and hopefully see it grow into the tool that I think it is showing promise of becomming.