So who's this Jimmy character, exactly? The bald mercenary, described as "the guy the other guys call when they need to finish the job," didn't appear in the original Mafia 2 storyline. In a nutshell, he's a fairly generic gun-for-hire type that's been betrayed by both the Irish and Italian organized crime rings, who he worked for in "The Betrayal of Jimmy."
The gameplay in Jimmy's Vendetta focuses on a few different tasks. They ultimately feel like side-missions that could have appeared in Mafia 2, but didn't. Various mission icons appear on your city-map, to be completed in groups of three, before a new set appears. A few different mission types including vehicle theft, property destruction, and assassinations utilize game's solid driving and shooting mechanics well; however, even though some of them take place in parts of Empire Bay that you didn't really explore during the main campaign, none of them feel as fleshed out as the missions in Mafia 2's main storyline.
It doesn't necessarily help matters that Vendetta's bare-bones storyline eschews the cinematic presentation of Mafia 2's main campaign in lieu of text-based mission briefings. You'll get a cinematic cut-scene at the very beginning and the very end of the content, but it's not nearly enough to elevate protagonist Jimmy or his plight with the city's various crime organizations into anything more than cardboard caricatures.
All of the missions in Jimmy's Vendetta are timed, and player performance is tracked and graded for online leaderboards. Things like completing a mission quickly, driving fast and power-sliding around corners, and shooting mobsters in the head will improve your level performance. So, if you're into replaying missions to improve your score (Grand Theft Auto IV: Ballad of Gay Tony-style), it's a nice bonus.
You'll probably get at least five or six hours worth of playtime from the extra missions included in Jimmy's Vendetta. How much you enjoy Jimmy's Vendetta will depend on how much you like driving around Empire Bay, of which there is a lot, and how much you enjoy the game's firefights. It delivers those things repeatedly, and with some instances of flair. If you're waiting for another cinematic experience and memorable characters in Mafia 2's world, or an extension of Mafia 2's abruptly-ended story, Jimmy's Vendetta will likely appeal to you as much as a pair of cement shoes.
You can also check out our review of Mafia 2 for more information about the core game.