Whenever I read about Jagged Alliance 2, at some point the 1.13 fanpatch/mod is mentioned. Mostly, people say it's the definitive version of JA2 as it adds and improves so much. For veterans, that seems to be nearly undeniably true.
But, what about players who have never played Jagged Alliance 2 before? Is it still advisable to use 1.13, or does it make things too complex, too extensive?

I've only played JA2 for a short time before and it started getting on my nerves with very slow progression, lots of missed shots by my troop and lots of lucky hits by the enemies in the first few hours. But, I'm interested in giving it another go, so I wonder if I should think about using 1.13 or if it's really only aimed at experienced players, anyway.

I would say, give original another try first. 1.13 isn't mere mod or patch anymore, it is more another version of the game already.

Well, maybe unless you already have read too much into details about the countless 1.13 features already - because then you may quickly find even more points of annoyance with the original. Inventory, squad size...

The greatest thing about 1.13 of course, it is the moding possibilities, with 1.13 you can quite easily adjust many aspects of the game. And most added or changed systems can be switched on/off (require starting new game though). In theory, you can even emulate 1.12 on 1.13 closely, but that not how it comes by default.

As to hit chance, one of 1.13 greatest features is complete rework of how firing works internally, making it possibility one of most realistic systems employed in isometric game, but perhaps surprisingly, one of main goals for it was to make you hit less (or more precisely, to combat 100% granted head shots with advanced and/or heavily modified weapons), and that what it does with default settings of 1.13, you will miss even more often. It can be tweaked of course.

Wondering about difficulty, 1.13 is, as probably can be expected, nominally harder than original, but it also offers you far more choice, both in setting up the game and in game. Things like massive counterattacks, enemy combat jeeps and tanks (in patrols, not only in Meduna) can be overwhelming if take you by surprise. At the same time you may find the play more smooth even.

To conclude, 1.13 is the way to go once you not satisfied with original in any way, and/or want more, the amount of options and added complexity may be confusing for starters, and if you interested in original gameplay you should try that first.

PS: JA2 is a slow game. While speed run in less than a day is possible, typical play trough is expected to take months. Both in in-game time terms and most probably real life.

Thanks for the in-depth reply!
What you've written actually sounds very encouraging, and especially encouraging me to play the vanilla game. It sounds a bit similar to UFO: Enemy Unknown, which at first seems (and for most people is) extremely hard and possibly frustrating, but at some point makes you so overpowered it becomes a cakewalk. Which I think is actually nice for a first playthrough, just not for repeated plays.

While comparison to UFO: Enemy Unknown isn't at all inappropriate on itself maybe (in several aspects even), the difficulty curve (from impossible to trivial as you progress) here is by far not that extreme, imho.

Well, you never will be able to go run-n-gun for sure. ;)
Tactics are mandatory. As is cover. And aimed shots.

It probably worth to note, as it is not enough stressed anywhere I think, game use somewhat scewed scaling, different activities use different scales to measure distance on tactical map.
The most extreme and important: for all aspects of firearms use one tile is thought to be 10 meters of distance despite visual representation and other actions may suggest something rather closer to one. That is deliberate design decision to accommodate to rather small map sizes, and meant to create more "Hollywoodic" close quarters shootouts, but may need to get used to, without understanding and internalising this quirk it can easily be source of major frustration as game pretend to be fairly realistic otherwise, and you wonder why you can't hit anything with your pistol across the street, but that street may easily be over hundred metres wide as far bullets are concerned. Even solid assault rifle may feel more like paintball weapon if you believe the visual scale.

Also, while present to same degree in vanilla, the headshots "problem" I mentioned was something 1.13 largely introduced on itself in early iterations, then sought to fix, and found that original system just hadn't have room and resolution necessary for everything that got trown in.

Enneagon has a much better understanding of the workings, problems and workarounds and such than I do but I will add my two cents anyway.
In my experience 1.13 is extremely better than vanilla. So much so that I cannot even play through Silent Storm (a PC game set in WWII which directly copies JA2 vanilla in game mechanics) or JA 2 vanilla anymore. However those improvements can mean a higher learning and difficulty experience for players who are not JA2 veterans. Inventory for example is much more realistic in 1.13 in terms of having body locations where various types of containers/bags/pouches can be place. For example there are certain types of holsters and first aid kits which can only be worn on the thighs and do not have space for large items (like rifles).

Combat is much better but in my experience you should turn off the "New Chance to Hit" system/option because it makes for much greater difficulty hitting targets (not nonsensically or buggy or anything like that but taking into account factors which most probably do not care for).

I would also recommend vanilla for the first time playing through for the reasons I mentioned above and also what Enneagon has stated. But subsequent playthroughs with 1.13 mod will be akin to playing a faithful recreation of JA2 released 10 years later and featuring expected improvements of a modernized remake, sans unnecessary graphics upgrades.

To be honest, I don't have that much 1.13 experience I would like to. But I have read substantial part of Bear's Pit instead :P

1.13 is still active and evolving, be sure you get the latest version, the "stable" is long outdated. If/when you decide to try other mods that require the "stable" 1.13 as base, use it with latest fix/additions/experimental from sevenfm.

As to NCTH, it always was a hot topic, at length [url=http://thepit.ja-galaxy-forum.com/index.php?t=msg&th=21487&goto=327477&#msg_327477]repeatedly and (there likely more, it just few I got quickly). As far I understood, "stable" indeed got somewhat twisted settings on it. Try the latest 1.13 redaction first, it might be not that bad [url=http://thepit.ja-galaxy-forum.com/index.php?t=msg&th=22757&goto=341752&#msg_341752]already, tweak it from there if necessary, but don't judge from very first fights on pistols (those might be very thought, but it supposed to go much better as soon you get better guns, scopes and experience on your mercs). Even then, OCTH is of course still valid choice if you so prefer.