Shadows of Mordor: Is it really like the Batman Arkham series crossed with Assassin's Creed?

It's more of a Batman than AC.

like the first guy said it is just like batman, but from my experience with batman if you put it on easy it allows you more reaction time to input ur fight actions

Combat is more of Batman. I do feel there is a bit of Assassin's creed in it. It's a lot more open world, unlike in Batman and Assassin's creed, where your stuck in cities for most of the game.

Is it worth it to pay this kind of money for a button masher?

my sincere appreciation for your generosity.

It has the Batman combat system, with the Assassin's Creed movement system (jumping, climbing, etc). But the similarities stop there.
It's very much worth the money.

If you suck at AC and batman you are not destined to suck at SoM. In shadow of mordor you can grind skills that can make you really OP, it also allows you to run from dificult situations.
SoM plays exactly like batman; it has an attack button, a counter, a dodge, a special attack move and adds a ranged attack to the combat. The combat is fairly easy IF you pick your fights.For example If you want to kill captain X that is in a fortress, you can't go in " guns blazing", the orc numbers will just kill you. You can approach this situation with multiple approaches like waiting for him to leave the fortress, stealthy assassinate him or create distractions along the way to move the orc troops from unwanted guard positions. With the nemesis system each captain has strength and weakness which can make the previous plans useless or the best course of action. For example a captain can be imune to stealth attacks or weak to ranged,etc.
SoM is easy because it (almost) doesn't punish you for dyeing or fleeing. When you fail captains will interact with each other becoming stronger (or getting killed by other orcs). It's possible that a captain if you fail so many times that it reaches a max level and have insanely unfair traits but that rarely happens since most weaknesses are relatively easily exploited.
Comparing SoM with AC is a bit farfetched. There's a lot of animnations that play the same but the "parkour" is completely diferent, in fact most locations only have certain parts that are climbable and not everything like the AC IV. in fact trying to run by jumping in the building is rarely as intuitive as the AC series.
Now that you have an idea of the game, i just want to "warn" you that SoM has only 1 activity and that's kill orcs, almost every mission and sidequest involves killing orcs, the only variations are either on the way you have to kill them or in the time you have to kill them.

The combat is the exact same as in the Batman Arkham games, albeit much more challenging and fast paced. Enemies will not stand and wait till you end your move/finisher as in the Arkham games, also more guys attack you at the same time. There is not much similarity to AC, besides opening up your map by climbing watchtowers. It has parcour stuff, but it's much faster and agile than any AC. Even your mounts can climb!

the combat is just like the batman series, beeing really good at it myself, i found this game really easy... i mean really really easy, capitans died left and right without even trying.

Yes the gameplay is like batman and assassins creed, but it's really good, imho. Totally worth the money!

You arent destined to suck at SoM though I personally dislike Batman games(I find combat kinda meh,dont ask why) but in SoM you will be given different tools to use and more or less you can go mainly for stealth/range approach with least amount of melee. One thing to have in mind though is that story missions are also a tutorial + Brand skill(take control over orcs) is block behind it so more or less I advice to do story and then go for 100% the game.
Now if you really suck then you gonna have a lot of fun with Nemesis system because for good players game is really easy.

Get it now. If you suck at combat don't worry there are plenty of ways to kill silently. Branding and shadow strike chaining is awesome.

No, its just a repetitive game that don't increase its difficulty as you play and only get easier and easier until you say "WHAT? THAT WAS THE LAST BOSS? BULLSHIT!"