Addictive, deep gameplay, lots of fun.
I can't say I'm a veteran of the series, since I only played Fallout 1 and 2 for a bit before giving up. Luckily I've gained a bit of patience and a love for role playing games since then, and Fallout 3 is just right for me.
I'm going to mention some of the shortcomings of this game, just to get it over with; animations and characters seem lifeless, weapons are perhaps a bit unbalanced (with some weapons (pistols, mainly) it takes multiple shots to a human enemy's head), and the AI leaves a bit to be desired.
Despite these faults, it is truly a great experience. While other games might be destroyed by such shortcomings, Fallout 3 has so many and deep features that it makes up for it, by far. The combat system is incredibly rewarding and fun, not to mention varied. The game gives you the choice to use switchblades, pool cues, mines, grenades, swords, all kinds of conventional weapons, all kinds of huge, unconventional weapons and even teddybears as means for destroying your enemies. The high-point of combat in this game is, however, the VATS-system. It enables you to target any enemy and body part in front of you, giving you the ability to unleash a massive amount of carnage with a lot of control. Watch you character grit his or her teeth while pulling the trigger of a scoped .44 revolver, blowing some poor Raider's head off.
There's more to Fallout 3 than gunplay, however. I've found myself scavenging ruins, raiding all sorts of bandit-camps, doing several missions for some loon in order to compile a "wasteland survival-guide", shopping for nice clothing, styling my character's hair, listening a radio station I "uncovered" solely playing violin-music, and much, much more. I think I'm going to have a hard time getting tired of this game, simply because there are so many fun and dynamic things to do.
There are heaps of fun situations in Fallout, and a lot of it is because of very good level design. Don't read too closely if you haven't played the game - a lot of these things are a lot of fun to discover on your own. For example, I tried to sneak into a slaver's camp to release the slaves and take whatever I could find. As I made my way inside, I came up on a shack the slavers used as shelter. When I tried to sneak around to try and get an overlook of the camp, I suddenly heard a thump by my feet. Turns out I'd tripped a wire, and three grenades had landed right next to me - I froze for a second, and the blast sent my character flying, instantly killing her and knocking her into a pile of rubble. In the middle of this camp was a huge cage built by various debris and destroyed parts, housing a gigantic super mutant. I didn't catch on until I'd killed everyone in the camp, but it turns out I was able to blow the generator powering the electrical fence keeping it inside, unleashing it onto the camp. That's the kind of surprising and fun situations the game places you in. :)
Well, I think I've written enough for one sitting... but I can say that if you like TRULY open-ended and varied games, Fallout 3 is highly recommended.