I've been an AVG free user for years. Intermittently I've used other, paid-for, anti-viruses but have always gone back to AVG. I barely know that it is there; my system doesn't seem to slow down with a full scan underway and it keeps out most viruses.
The paid for versions add more functions, some of which are definitely very useful, others depend on how you use your computer. For instance, a useful feature is the anti rootkit tool, it keeps out the most pernicious, nasty threats that the free version doesn't, while the game-mode may be handy for gamers, others may get little use from it. The link scanner is also handy, finding dangerous links in the results of internet searches.
I've had little opportunity to find out how well phishing protection, identity protection and anti-spam work, it may be that they work so well I haven't noticed them or simply that no threat has appeared. A quick look in the advanced settings shows a host of processes monitored though.