Numenera is a really simple game at its core. It's all about setting difficulty levels that the players must overcome on their d20 rolls. The game uses very few dice to accomplish its mechanics, which I think it really excels at. You only need a d20, a d6 and a d100.
This simplistic system becomes quite interesting once you start involving your character. Each character is described by a single sentence: "I am an <adjective> <noun> who <verbs>". The adjective is a trait like "Tough" or "Clever". The Noun is your "Class", as it would be described in most other (d20-based) RPG systems. Numenera has three classes: Glaive (warrior), Nano (wizard-ish) and Jack (rogue-ish). The final, and to me the most interesting, trait of your character is your verb, your descriptor. It can be something as mundane as "Leads" or as strange as "Rides the Lightning".
We have 12 adjectives, 3 nouns, and 29 verbs for a total of 684 distinct character presets in the core book alone! I think the character creation is one of the system's main strength. About the art in this game: it's great! This game has really high production value, in line with most of what WotC produces these days.
The dice mechanic deals with low numbers, and actively strives to lower the TN before adding greater modifiers. It's great on paper, but sometimes I feel like it does noting to actually make the game run faster. PvP is something this system handles quite poorly, as the dice mechanics were designed for one party rolling all the dice (the GM normally doesn't roll any dice what so ever).
A very interesting mechanic of Numenera is the "GM Intrusion" mechanic, where the GM is essentially allowed to screw over a player at any time of his choosing. The downside? The GM must award the player XP for the hideous deed. Players may spend XP to avoid this ill fate. Honestly, I've stolen this mechanic for use in other games.
Numenera's setting is The Ninth World. It's a world with quite a lot of quirks and strange history. It is Earth set in the far far future (about 1 billion years). Technology of the previous eight worlds are not indistiguishable from magic. The wonders of the world are its main strengths, with the legendary artefacts from the previous worlds known as the "Numenera" are the most profilic. There are two main forms of Numenera, Cyphers and Artefacts.
Cyphers are one-time use items that might have great effect. In one game I gave a player a nuke. But they really needed it. Some of the monsters in this setting can be quite horrendous (for better or worse)! Artefacts are another kind of item. These are more like your standard wands, staves, and other semi-permanent magical items. Instead of a set number of charges, artefacts have a depletion roll. It means that you have a chance (like 1-2 on a d100) to have your use of the item be your last. This mechanic is really good for minimizing paper work and I've started using it for wands and staves in my D&D games as well!
If I have to nitpick about the setting, I'd have to say that the political situation among the nations are a bit stale and some of the nations are also somewhat homogenous. But the Numenera are awe-inspiring and stimulates your creativity.
All in all: would I recommend Numenera? Definitively. I think it adds a lot of different elements to the gaming table. The system and the setting is very good for bringing in the weird and extraordinary to your tabletop gaming. Some of its mechanics are worthy of stealing for use in other games, while others are mere novelty. Character creation is one of the system's greatest strengths. But one might want to change things up in the nations if the GM wants to have a politically inclined campaign. This system is not for you if you expect any kind of PvP action between the players. Tactical combat is poorly supported in this system, favouring a cinematic approach instead.
[5 of 5 Stars!]