It is impossible to ignore the fact that all Megaman games follow a common formula. With very little exception, almost every core Megaman title follows the same structure: beat eight bosses in any order, steal their powers, then challenge the final dungeon, where Megaman has to defeat all of the bosses over again before challenging the final boss. With some variation, that formula has remained the same for all 24 core Megaman games (which includes the original series, X series, Zero series, and both ZX games). Yet, this formula has never diminished my enjoyment of the Megaman series. If anything, the Megaman formula is one of the key elements that makes the Megaman series so successful.
The basic formula of the Megaman series has quite a few inherent strengths that have helped make it so enduring. The basic premise of letting the player tackle the game's stages in any order allows for a lot of freedom. If the player finds one stage too difficult or frustrating, the player can just leave and return later with more experience under his belt and new abilities. The free selection of stages also gives Megaman games a high replayability factor, because a player can get a different experience out of tackling the game in a different order. Personally, I greatly enjoy the process of working out which bosses are easy to kill first and which bosses are vulnerable to which weapons. It isn't something that can be experienced in a linearly structured action game.
Furthermore, just because the Megaman series has had the same basic formula for over twenty years doesn't mean that the series has gone stagnant. From the get-go, there has been a lot of experimentation, elaboration, and incremental improvement done on the game structure. The expansion from six bosses to eight, hidden upgrades, introductory stages, mid-game story stages, mini-bosses, optional missions, mission objectives, side-quests, stores, more than one controllable character; almost every iteration of the series has added something to the formula. Certain games have also experimented with radical variations on the formula, such as Megaman X5, which added a time limit to the game, alternative means to reach the final dungeon, and multiple endings. Later Megaman games such as Megaman Zero also used a mission based structure that both stayed true to the original formula and allowed for more varied stage designs. So while the basic formula has stayed the same, it has also evolved considerably over the years.
Being formulaic also doesn't mean that Megaman games lack plot, characters, or fun gameplay. Over the years, the game designers for the Megaman franchise have learned how to write interesting stories that can be presented effectively alongside the game's structure. The Zero and ZX games in particular manage to tell good stories with fun characters without breaking down the basic formula of the series.
If anything, the formulaic structure of the Megaman series gives it certain strengths. The Megaman formula is familiar to anyone who has played a Megaman game. For me, there is an element of nostalgia when I play a Megaman game that harkens back to the days when I played the series as a child. There is something familiar and comfortable about the series. It is no wonder that the Megaman series has cultivated a strong following over the years that became very excited over the recent release of Megaman 9.
Even though the Megaman series is very formulaic, I don't see that as a bad thing. As a long as a series continues to incrementally improve on itself, staying true to its roots can actually be a strength.