Rage is very much a traditional FPS with some minor open world elements. The basic gameplay structure is that you arrive in a settlement or a town, a set of missions are available (in the early stages of the game there is often only one, but later on there may be 2 or 3 to pick from), you have some choice over the order you can do some of them in and some are optional (but many missions don't unlock until earlier ones are completed, and it's often the case that those where order is unimportant are also those that have no effect on the main story line) and you go do them. Within the scope of each individual mission the game is quite linear: you progress from area to area, kill bad guys in various interesting and fun ways, pick up some stuff, get to your objective, then make your way back. Don't let any of that fool you into thinking that you'll get any kind of hybrid style though; it's maybe 75% to 85% FPS and the open world elements rather tinge the game instead of appreciably influencing it.
The combat is a mixture of cover system and old-school run-n-gun, with different tactics being more suitable to different enemy types.
There are no skills, no experience system, no getting stronger. Instead you can build items that increase your health, buy better armour and get better weapons. In that sense I feel that it's more similar to a Japanese RPG than to a European/American one (I've compared it to Final Fantasy 7 in the past). But again, it's a tinge, a shade, a hint.
The weapons are pretty much the standard id Software fare: shotguns, machine guns, rocket launchers, grenades. There is a crafting system that can allow you to build better (or just different) ammo types for them, and you do get to make your own choices on which to use at any given time - just like a traditional id FPS.
There is a one-time-only option to select an outfit from 3 options that give you different bonuses, but aside from that armour comes in light, medium or heavy varieties - again, just like a traditional id FPS.
So would I tell someone to buy it? Yes and no. If you liked the Doom and Quake games, in particular the single-player campaigns in Quake 2, Doom 3 and Quake 4, then you'll probably like this too (unless you're a part of the hate campaign against it in which case you'll hate it anyway). If you thought they were OK but needed a little something extra to make them more interesting, then Rage might just offer the appropriate something. If you're an RPG fan but fancy dipping your toes in the traditional FPS waters, then Rage might be a better gateway than something like Doom 3 or Quake 4.
Some other advance warnings that it's fair you know.
Bugs in AMD graphics drivers are still being worked out; if you have AMD hardware it's best that you wait until the 11.11 driver release (which is supposed to be this week) and check out if it really does fix them - AMD have twice messed up a driver release for Rage so caution is advisable. If you're on NVIDIA (or a console) then none of this applies.
There are some gameplay bugs that bite some people but don't even happen to most. A future Rage patch should resolve them, otherwise you may need to go back to earlier game saves in order to continue.
If you have preconcieved notions of what "good graphics" means then you may need to check those notions in at the door. Rage's graphics are excellent but they are also very different to every other game on the planet, and this has been a difficult pill for some to swallow. Texture detail up-close was sacrificed for the ability to have a completely uniquely textured world (and it may need a few hours of gameplay in order to fully appreciate what you're looking at) and insanely high polycounts. If you value up-close texture detail above everything else then you may be disappointed. If you're more open-minded on these things then you may actually find it difficult to go back to other games which have repeating textures and lower polycounts - it's one of those things that it may take some time to notice, but once you do notice it you'll realise that it's another point-of-no-return innovation.
You're probably used to longer playing times, and Rage - as an FPS - may disappoint you here too. It actually has a very good playing time for an FPS - you can get maybe 30 hours out of it on a first playthrough if you don't rush the game, take your time, explore. Again, metrics that apply to other games don't apply here, so check in your preconcieved notions at the door. As a 21gb install you may feel that even 30 hours is short, but the reason for the large size is that unique textures obviously take up a lot more space.
I rate it very highly; definitely id's best single-player game to date, with a compelling visual style that (if you're prepared to drop notions of "texture detail up close = good graphics") is totally unlike anything you've ever seen. Given the gameplay bugs (which maybe 99% of people won't even experience anyway) and a disappointing ending I'd drop it to a 9 out of 10.
I'd have a hard time definitely recommending it as a purchase to a Fallout or Elder Scrolls fan (but then I wouldn't recommend a Fallout or Elder Scrolls game to a Doom or Quake fan either). As an FPS the play style is sufficiently different to what you might be used to. As an old-school FPS it's also sufficiently different to what a modern FPS fan might be used to, even. If you are intrigued enough by it and if you're a console owner I'd definitely recommend a rental and a few hours play first, then make your decision.