I've been a regular Ubuntu user for a few years now. At home I run two systems, one is my laptop which I use for browsing the internet, watching youtube, editing photos, etc. The other is a dedicated server/media center set up to my tv.
Coming from a Windows background at first it took a bit of time to adjust. I'd like to say that Ubuntu is perfect out of the box, but there are some things you need to be aware of.
First off, don't expect to be running any of your favorite windows software. More often than not there are linux equivilents that will run much better than trying to emulate a windows enviroment. Linux does not natively support windows software so work arounds are required which can be very difficult for a novice. Keep in mind though that many companies are now offering linux versions as well, such as Firefox, Opera, even some Google apps are available for Linux.
Installing new software such as Flash as mentioned before is normally fairly easy. The software library is great and easy to use. It's so simple to just type in a few key words for the kind of software your looking for, and have a whole list with descriptions, with a couple mouse clicks you can have the software installed no problem. The other nice thing is Ubuntu will streamline all your updates into one central update tool, so you won't have a pop-up for every program that wants to download updates.
Viruses have not been a concern of mine in years, most are written to attack windows or mac's so your pretty secure in that sense, however don't get a false idea of security, you still need to update regularly, and should employ additional measures if your data is particularly sensitive.
All in all it's still going to be a steep learning curve, and while any user can easily jump on, browse the internet and use the software, if you do need to troubleshoot or have hardware issues, your going to find yourself googling solutions and working in the command line a little bit. But like everything else, this comes with practice.