While I haven't specifically played the Horizons pack, I have tried some of the mods used in it for another project. So here's my thoughts on what I have tried:
ATG: Beautiful, fun. I really like this one. It's Volcanoes put everyone else's to shame. However, I did have problems getting it to play nicely with other worldgen mods over a long period of time.
Biomes O Plenty: Also fun. Adds a lot of blocks/items that have no purpose beyond decoration. Can make certain tasks too easy or too hard, which can be a concern for a more focused/goal-oriented pack. Still needs more development given the significant number of builds it has undergone without a stable/recommended release. I look forward to the realization of a stable Biomes O Plenty + ATG world.
Ender IO: I was hesitant about this mod initially, but it is very nicely configurable and does everything it tries to do well. Uses similar modded ores (copper/tin/etc) which makes it very easy to balance its entry point with other mods.
Farlanders: I love this mod. Maybe it's all the nostalgia regarding minecraftforum fiction written about encounters in the Farlands, but it just seems like a proper addition or natural extension of Vanilla.
Gany's Nether, End, Surface: Very practical and brings a lot of fun little touches to the game where each bit is configurable and plays very nicely with other mods. It's like someone made dozens of useful little mini-mods and got them all working together happily. If I do more with modding, this is the approach I'd want to emulate.
Metallurgy: I only tried this on one world, so it deserves more of a fair shot considering all the stuff it does.
Pam's Harvestcraft: Fun, but so much stuff! It was too much for me for what I do. But that'd be quite appropriate in an "All the things" kind of environment where more is more. It's all shaped/shapeless recipes, so it will work very well with various auto-crafting solutions. Have you tried to make all the pie?
Roguelike dungeons: These are brilliantly executed. But then I love dungeon crawling games like Rogue. The only problem I had was the same as mentioned. They're pretty big and spawn pretty frequently. So I opted to make them more rare and smaller to help with performance and "conquerability". Eyamaz (or anyone else that wants it), try this if you want similar:
Code:
levelScatter=12
levelMaxRooms=10
spawnFrequency=20
levelRange=40
doNaturalSpawn=true
Thaumcraft 4: Very different from TC3. I like what they did with the wand. Not really a huge fan of the taint stuff, but could learn to live with/deal with it as it seems there's more powerful tools available for handling it. Not really my thing, but then TC3 wasn't either. Like Pam's, good for an "All the things" kind of environment. Or maybe a polluted wasteland type server with the right texture pack or settings in Biomes O Plenty. Seriously, doesn't it weird anyone else out that the only mod that does industrial pollution well is a magic mod??
Utility Mobs: FatherToast has a lot of fun mods and this is no exception. Build yourself an army of various golems to protect your stuff or build turrets like tower defense games. They can even be configured to recognize your friends and kill your enemies.
Armor Status & Status Effects: I love having information so I love having these. While I could see how someone might complain how they might be slightly immersion-breaking, but so are minimaps and such. It's convenient to know how long an effect will last and how much durability is left on your armor/tool without having to open your inventory or squint at a damage bar.