I've recently tested out several different AV products and evaluated them based on usability, efficacy, and performance, among other criteria, and FortiClient seems promising, though a bit "clunky," which is mainly due to the use of windows instead of notifications to alert when there's an issue. The main problem with this is that almost every time, the window fails to take focus, so it remains hidden behind other windows leaving me wondering what's going on until I see its taskbar button. So when testing by dragging an infected file from a network drive onto the desktop, for example, nothing seems to happen. I don't know how much of this is due to FortiClient and how much is due to Windows, since I've noticed issues with this much more in Win10 than in previous versions, but FortiClient does it far more than any other program. In fact, not only does the main alert window almost always pop up behind other windows, but just now I even had the pop-up window asking if I want to add a file to the exclusion list open behind the alert window, and no matter how much I try, I can't get it to show above it, so the only way to interact with it was to move or collapse the alert window so I could get to the exclusion one, then move that one out from behind it.
Then, when I look at the window to see what's going on, the columns, as expected, only show so much info and then it gets cut off, so the whole virus name and item location isn't visible. The problem is there's no tooltip when hovering over it, and double-clicking it doesn't open that item in a separate window for further analysis, so the only way to see the whole name/location is by increasing the width of the column(s). And the window size itself is fixed, so just widening that isn't even an option. Also, there's no way to copy the info or perform a search on it, making it much harder to look it up online.
Then, once a certain file has been quarantined once, repeated attempts to use it (e.g. if I try to move it from the network drive to the desktop, FortiClient quarantines it, and I try to move it over again) don't trigger the notification window. I realize this is an unusual and unlikely scenario, but an AV program should alert the user each and every time it blocks something. When I mentioned usability as being one of my testing criteria, that includes that there should never be an instance where the user is trying to do something and having it fail, not knowing why, only to have it end up being the AV software silently quarantining/deleting/etc in the background.
Other than these few issues, I find FortiClient to be very seamless and easy to use. I like how easy it is to remove an item from quarantine and add it to the exclusion list. One more thing, which is off-topic, but I'll throw it in here: I was disappointed to find the scan speed to be rather slow, and with no improvement on subsequent scans, but that's not a big deal.