In the interest of not revealing spoilers to those who don't want to read them, I'm doing things a little differently with this post. I'll do the overview, then share my thoughts, and only after that share my experience with Risk Legacy.
Risk Legacy is an enhanced version of Risk, the main new concept being that the game and game components will change permanently as games are played, some being modified by the players, like marking the board and adding new elements to it, some being revealed bit by bit as certain conditions are met and players are instructed to open some sealed envelopes. Risk is a mass market game, so I won't describe the basics. Risk Legacy uses the basic earth map. Its winning condition is based on victory points, i.e. controlling headquarters, completing missions and trading in resource cards. There are different factions in the game each starting with different special abilities, and additional special abilities can be introduced as the game is played. The game is expected to stabilise after 15 games, i.e. no further modifications to rules and game components.
The concept of letting players modify the game permanently is a novelty, a gimmick, but it is fun. Aspects of the game being gradually revealed is fun. There are new rules, changes to rules, and new gameplay elements. Cities will get added, territory properties will be modified (e.g. becoming harder, or easier to defend), faction abilities will change, continents will be named. There is a story line developing through the series of 15 games being played, which is aligned with the rules changes. However you need to keep in mind that this is still Risk, just with some improvements. It is still about breaking one another's control of continents over and over, accumulating many resource cards to exchange for troops to make big attacks, and dice luck being able to spoil the best laid plans. But it is also about waiting for the right moment to strike - after you have enough resource cards to swap for many armies and before your opponent strikes a fatal blow. It is also about trying to appear weaker than you are, and persuading others to fight. For players who are familiar with the game, there is careful manoeuvring and positioning before launching a major attack. It can be tense.
One concern I have with the game is it seems to encourage players to start from the same locations, which might make each game feel similar. At the end of a game, you get to add some good stuff to the map. Very good stuff if you win, moderately good stuff if you lose but are not eliminated. Once you add a city, in future games, only you may place your HQ in this city. So there is incentive to start here, and in subsequent games you are somewhat encouraged to add more good stuff in that general area, which compounds the problem. I'm not sure whether this is commonly experienced. It happened in the campaign I played, and I intentionally did weird stuff, even some not beneficial to "my" starting area, hoping to mix things up a bit and prevent the games from becoming scripted or static.
Risk Legacy has been a fun journey of discovery so far. There were some stale moments, e.g. some games got into long stalemates with nobody being able to control any continent, but those tend to be earlier in the campaign when the game was still quite bare. As more elements were added, this happened less. Our games were all 3-player games, and they reminded me of Romance of the Three Kingdoms, where the three kingdoms maintained a delicate balance of power, always keeping one another in check. There was much cajoling, pleading and convincing others to fight. We were all sneaky. Sometimes A intentionally refrained from attacking C to conserve his armies, forcing B to do it on his turn which is just before C's turn, because otherwise C would receive a big continent bonus. Alliances formed quickly, when the third person threatened to win, and also dissolved quickly, when one ally found a convenient time and location to launch an attack. All in a day's work.
Risk Legacy is suitable for casual gamers, because it is not that different from regular Risk. For gamers, it is still fun as long as you set your expectations that it is not really that different from regular Risk.
I realise I have already described what it feels like when playing the game, so this will be just the spoiler section with more photos.