Personally I think Monster is great at what it is: a massive pack of very cool mods that allows the player to do what ever they want to do (or pretty much there about) with modded MC without 99% of the fuss of making your own pack.
I played the 1.4 mods and stopped playing when the 1.5's started coming out as they seemed to be just a reiteration of the earlier versions. But when Monster came out I finally managed to wrangle my friends into playing a private server with me and we've been enjoying Monster since 1.0.3 like FTB is a completely new game again. Everyone can do whatever they want, I've enabled "All The Things" and I haven't heard any gripes yet (except the moans from the inevitable reset when it goes stable).
Honestly I think Monster is only getting a bad wrap because it's in a transition area in minecraft in general. Minecraft is more popular, better managed, and just a much bigger beast than it ever was in the past. It's got more mods than you can shake a stick at now, not just the core mods of IC2 and Redpower of days gone by. It's got more frequent updates as these mods/packs that were once done on an individual level are now starting to turn (by necessity) into team efforts. It's just turned into a beast of a genre game. All this and trying to package it all into a nice little (okay it's big) ball of cozy modpack puts lots of strain on everything, those creaters of the mods and packs, server admins, and even players. It's almost too exorbitant. Monster is just what it's name implies!
However, as a flagship pack, I also have to say it's great for all the same reasons! It gives everyone the options to do what they want out of the box. Think something's overpowered for a public server? Turn it off, simple. Don't like that it doesn't come with your favorite mod from that other pack? No problemo, add it in with the universal config, again simple. Don't like your neighbor gloating over how he has stacks and stacks of diamonds from a quarry in his ME storage? Make a nuke and detonate his base... probably only on my server, but still simple!
The only reason I really see it being a bit lacking (I've talked it over with my friends whether to switch to DW20 or Horizons a couple times) is that there are the fore-mentioned packs that are also laid out as if they were flagship packs. They both have strikingly similar core mods and loads (I mean loads) of gameplay enhancing mods. I know Horizons was supposed to be the "Lookie what this mod does" pack but come on, it's a full blown pack compared to all the other adventure/survival packs listed. The same is true with DW20's 1.6 pack, it might as well be Monster with a half dozen mod swaps and config tweaks. They're WAY similar, and that almost definitely takes away some of Monsters roar so to speak.
I really think that with the versatility you guys/gals have put into the universal config that it's almost a shame that the three most similar and probably, in my opinion, best packs for general gameplay are feeding on each other's fanbases instead of just being combined or culled down to be separate like they were intended. Horizons shouldn't have most/any of the core mods that it shares with Monster/DW20 for obvious reasons; it's a demo pack. And DW20 should in respect to Dire and his crew be just a reconfigured Monster. Monster should be the penultimate pack with every mod under the sun (within reason), ready for the server admin to cull it down to what they want or run it full out without an issue. That's what a flagship is in my book.
Overall, you're all doing freaking awesome, and I am immensely enjoying the experience
And I've been lurking for ever and this made me post, shame on you guys!