Introducing a new main character into an established and popular franchise is very tough. The poor fan reception of Raiden from Metal Gear Solid 2 is proof of that. However, I think Capcom did a very good job of bringing in Nero and selling him as a cool character. I think that this is in large part due to how the developers went out of their way to emphasize that Nero is a very different character than Dante, as opposed to being a clone or a replacement.
From the beginning, it is obvious to any long-time fan of the Devil May Cry series that Nero plays very differently from Dante. While he still uses both a rapid-fire gun and multi-hit sword combos, Nero's specific combos and special moves work very differently than Dante's. Furthermore, instead of copying Dante's styles or ability to weapon change, Nero uses the Exceed system and the very powerful Devil Bringer. The Devil Bringer in particular differentiates Nero and Dante, since it forms the core of Nero's fighting style, especially against bosses. In fact, even though Nero and Dante will both fight the same bosses, it is noticeable that very different tactics are needed. While Dante can count on superior mobility and long-range firepower, most of Nero's strategy focuses on finding opportunities to hit enemies with his Devil Bringer's Breaker attack. The two character's Devil Trigger abilities and appearance are even significantly different.
The Devil Bringer is certainly a fun weapon, particularly since there are custom animations when Nero uses it against bosses or various normal enemies. Nero's Exceed ability, which let's him charge up his sword to get more powerful sword attacks, allows for some pretty spectacular combos as well. Unfortunately, it is pretty obvious that these powers are new additions to the series, and thus need some more refinement. Despite being the power that distinguishes Nero the most, there is surprising little to do with Devil Bringer. The player can pretty much just use Snatch to close the distance with the enemy and Breaker to unleash a punishing throw. While Nero later on gains the ability to hold an enemy as a shield, that rarely seems to work like it is supposed to and ends up feeling like a minor side-note. I would have liked to see a lot more functions and upgrades for the Devil Bringer, such as the ability to throw an enemy away from Nero, choke an enemy, or something else like that. Exceed can also use a little more work, since I ended up forgetting about it more than half the time. While it is possible to easily build up a single level of the Exceed gauge by using Exact, that isn't enough to allow use of the more powerful Exceed 2 or Exceed 3 abilities. And while Exceed attacks look cool, no enemy seems to have a particular weakness to them, so there often isn't a whole lot of point to building up the Exceed gauge. Still, Nero's abilities were well-executed enough that I actually missed having them when time came to switch to Dante.
The game developers also did a pretty good job of differentiating Nero and Dante's personalities, even though they are both generally cut from the same mold. Even though both characters fought many of the same bosses, their banter with the bosses and general behavior is very distinct. While they are both cocky and confident, Nero tends to be short-tempered, straight to the point, and serious, while Dante tends to be relaxed, joking, and generally more of a show-off.
Finally, I think the story of Devil May Cry 4 did a good job in making Nero feel like an interesting and important character. Metal Gear Solid 2's big mistake was in hyping Solid Snake as the main character in most of the promotional material for the game, making him the starting playable character, and then doing the surprise character swap to Raiden a quarter of the way into the game. That process made Raiden into an unexpected and unwanted replacement for Solid Snake. In Devil May Cry 4 though, Nero was very much at the forefront of the trailers and demos for the game, so players went into the game expecting that there would be a new playable character. Making Nero the starting character also gave him a chance to establish himself and his role in the story early. The end result is that even someone like me, who is a big fan of Dante after playing the previous games in the series, ended up liking Nero a lot.