Seriously? I never knew this...always thought they were just a best of for every two games (aside from V).
Seriously, you thought that? No. Very much no. Not one NES stage design exists in any of the five Game Boy titles. No boss rooms are exactly replicated either. Every level is new. Each game has two halves, with four bosses each, with some kind of boss or fortress in the middle between the two parts. (Well, except the first game, but that still does have four bosses each from two games in it; the second four just don't have stages.) Then the final Wily levels are at the end. There's also one new boss in the Wily sections of each game.
So yeah, each of the first four games has four bosses from one Mega Man game and then four bosses from the next one, but you play them in a different order -- for instance, in Mega Man in Dr. Wily's Revenge (MMI for GB) you play Fire Man first and Cut Man last, of the first four. And the levels only resemble the NES stages in graphical themes, the actual layouts are, as I've said, entirely different. And even some of the themes changed; Cut Man's level isn't all that much like his NES one.
Also, the GB games are tough. Some of the NES games are hard, 2 (in Expert/Japanese mode) and 3 particularly, but MMI, III, IV, and V on the GB are all fairly tough games. II is laughably easy and is easily finished in a couple of hours the first time you pick it up (and it has some of the worst music ever in the franchise, too, and bugs!), but the others are a challenge. MMIII was a hard game... some of those stages are just crazy! I did eventually manage to finish it, but it was tough.
Oh, and the shops were introduced on the GB too. MMIV is the first Mega Man game with a "money" (P-chip) system and a shop you can go to between levels to buy things in. That later re-appeared in MMV and MM7-10, but it was first in IV. Yes, there's a reason why MMIV and MMV are two of the most expensive US-released Game Boy games... they are somewhat uncommon, and people know that they are great. That's one reason why I'd advise starting with the first and third games; they're much cheaper, and are definitely worth playing. I think III gets overshadowed by IV, often, but it really is a very good game... III through V are some of the best 8-bit-style Mega Man games ever made.
In comparison, again, the two Mega Man Xtreme games DO reuse SNES levels. The only original stage designs in the MM Xtreme games are the Sigma levels; all Robot Master stages in each game are direct copies of the SNES designs. At least those Sigma levels are new, and some are hard, but still, it's a lot of rehashed stuff. They do an impressive job of making the SNES stages work in 8-bit and with two buttons, but they are just ports. And yeah, the Xtreme games are decently good, but they're no match for MM III through V, and I'd rather play MM in Dr. Wily's Revenge too because at least it's an entirely original game.