My recomendation is based on: - I played the game on Hardest difficulty, with the "Realistic Rebalance" mod installed, that made stealth combat much more enjoyable and improved many things in the game. Many thanks to Killatomate for his fantastic modding work. - I played JA2 back on the '90s (original and Wildfire), and am a big fun of the original games, which I also recomment to everyone.
In general: JA - Back In Action (moded) had me staying awake for nights on the row, with many fulfilling and rewording moments when my combined attack plan succeded in nutralizing six-seven enemies in a matter of seconds, when my stealth inflitration commado managed to get more than ten kills before remaining enemies even knew he was arround, and when my 'Boom Boom' squad cleared the batlefield with a rain of granates and RPGs.
The "pause-able real time" mechanic is not the "turn based" I loved in the older JA and X-com series tactical games. But it is more 'realistic' and can result in really nice momments . I am enjoying playing also JA2-Wildfire simultaneusly with JA-Back in action, since the different mechanics provide two fantastic but indepented game experiences.
One-time purchase cost of the mercs, and AI insisting on getting only two assoult routs (so no more than one-two sectors need defending), means that money soon will not be a problem. So, I got overenthusiastic and got more than 24 merks under my command (something almost imposible in JA2). At that point, micromanagment of their re-equipment took more than the combat itself, and I started getting a bit frastrated, even though I liked the huge variety of weapons available.
A critical fumble of the developers: They didnt include the option to create a character to be your avatgar in the game. I got angry on them, but afterwards I got over it...
Leveling with skill points and no training option as it was at JA2: As a result the local free-merks can never be super-merks. So I use them as packing-mules, while at JA2 games I always had the main squad consist of my character and the local fighters.
DO NOT PURCHASE THE "xxx Specialist Kit" DLCs. They just provide at the start of the game one above-average weapon and a one-time-use armor-helmet set. The variety of weapons and ammunition in my gamming, ment that really soon I had other alternative options, so the money spend on the particular DLCs were not justified.
Personally I value more the JA2 (Wildfire and original), so I propose you start with them, and afterwards go into Back In Action.
I finshed the game but this game has really bugs, enemy soldiers can consistently hit my mercs from two screens away while my top marksmen seem to be 'out of effective weapon range' to fight back. its not hard its bugs we must play to enjoy but this game maked me really opssit the enemy team can kill you from 3 k.m far but you cant kill him from 2 meter with best merc with the best wepone thats sucks