VideoGame ARRRRRGGGHHH
Mega Man Zero. Four titles full of Nintendo Hard.
To specify, the Zero series takes place some time after the X series. A utopian human society, Neo Arcadia, has finally been built, at the cost of Reploid individuality. It's far too easy for any Reploid to be considered a Maverick now, so it's all a small Resistance force can do to avoid being wiped out. Their leader, human scientist Ciel, gets wind of the location of the sealed legendary hero, Zero (now sporting a new and not really improved design, in my opinion), and she goes to release you, and the series goes from there.
The plot, while fantastic in many ways (being even darker than the X series), shouldn't distract you from the make-it or break-it deal for you when considering the Zero series: the gameplay. Dear God the gameplay. The gameplay is... fantastic. Seriously, it controls well, and everything does exactly what you want it to. The issue, as previously mentioned, is the difficulty. This series isn't for the faint of heart. It isn't exactly four Mega Man & Basses back to back, but almost every boss has multiple health bars, every stage is full of spikes and bottomless pits, and if you expect to get through anything without dying, YOU. WILL. PRACTICE. This holds true for every game, to include the fourth.
To make things easier, you can use the Cyber Elf system to boost your health, add combos, and some even eliminate all the spikes in the game or half boss energy. But if you do that, you can kiss your A rank goodbye, and you'll need it if you want the EX skills. EX skills aren't necessary or useful half the time, but I'm a massive kleptomaniac, so the series is torture for me.
Musically speaking, the games are amazing. Some of the best the GBA has to offer. 2 & 4 in particular have really, really goood tunes. Just try and forget "Awakening Will" or any of its variations. Go on, try.
Overall, the Zero series is incredible, but expect to pull your hair out in all but the first game if you want the EX skills. The only really bad thing I can say about the series as a whole is that it has tons of unskippable dialogue. Other than that, grab Zero collection on the DS now. You won't regret it.
VideoGame A Hardcore, Satisfying Adventure
Mega Man fans usually holds the Classic Series as the crown jewel, and maybe the first half of the X Series; I cannot argue with that sentiment, but the Zero Series greatly impressed me despite its flaws.
Gameplay: People consider this the hardest Mega Man series, and they are half right. Zero has a fraction of health compared to bosses and level design leaves no room for mistakes. However, Zero one of the most agile protagonist in the original timeline; he can dash attack and he is capable of charging his both his saber and buster (take that, X5 Z Buster!). With a powerful long range and a powerful melee weapons, you can either opt to blast foes from a safe distance or charging and slashing without giving the enemies' room to strike back; the latter is more difficult to pull off but more satisfying. All 4 games suffer from Scrappy Mechanic like level grinding. I recommend having a save file where you have 2 Subtanks and unlocked charged saber and shot on New Game+. These game are more fun to play on the second play through.
Story: The Zero Series is lauded by many fans (me included) as the best story in the franchise, and not without reason. The Darker and Edgier story shows the consequences of X's and Doctor Light's ideals, as well as the treatment of humans toward robots. The games admirably ditches Status Quo is God and let the story move forward and wisely end at a bittersweet Grand Finale. Characterization is also superb, with both protagonists and antagonists greatly benefit from character development. My only complaint is the retconning of Zero 1 villain makes Ciel improbably smart at the age of 6-7. I also dislike how Capcom gives Zero amnesia as an excuse to make more X games (that have increasingly off the rail plot).
Music: While the stage themes are largely forgettable, all 5 opening stages (Zero 4 has 2 opening stages), final boss themes, and ending credits are powerful pieces. Just make sure you listen to the Remastered tracks.
Verdict: Due the the series' difficulty, not everyone will get to experiences the fantastic swordplay and plot. This series is geared toward veteran X players and can come off as a bit "elitist." If you are a patient gamer, give it a shot.