I've got my 2 handed up to 100 and I have the option to make it legendary. When it resets to 15 is that a normal 15? Will I have to work it back up or is legendary 15 more like 115?

It's 15, back to the beginning, your player barely knows how to hold a two handed weapon.

And I would want that because?

Nobody says that you must do it.

You can sacrifice your current skill level for the opportunity to gain a higher overall level while training the same skills (e.g., you want to reach a higher character level but have already reached 100 in all the skills your character focuses on). That will allow you to fight stronger foes, do certain quests, gain more perks, and get better loot.

You'll be "refunded" the perk points you invested in the skill, so that you can invest them in other skills or save them for when the skill increases again.

You don't lose your equipment, spells, or stats (health, magicka, and stamina), so you're still stronger than an out-of-the-box character.

This - once you get to 100, no matter how much you use that skill, it won't increase, and so, using it won't contribute to leveling up. I used to concentrate on two or three essential skills, but as they topped off at 100, the character couldn't level any more. At this point, you have two options: start using different skills which are still at 15 or 20 so you can level or you can legenderize the skills that have peaked. If you decide to use different skills, the PC become a different character which is a real issue for some folks. If you legenderize, you can keep the character the same but you'll have to ignore how they suddenly forgot everything they knew. Some people can look the other way, some people can't get over it so they don't legenderize.

Subtle, but fundamental, issues at this point. The good thing, as already mentioned, is that you don't have to do anything. I've become more inclined to keep a skill at 100 and not level as much.

Legendary resets the skill back to 15. I'm pretty sure the game gives this option so the player can continuously level up. For example, if every skill was at 100, the player would no longer be able to increase any skill, therefore the overall level wouldn't increase either. All the upgrades in that skill will be reset as well, and the perk points you invested will be refunded, and you can use those points on other skills.

In my opinion, this legendary option isn't something I would do. There's a lot of other skills to increase.

Weapons are fine....just takes a few more swings to kill stuff......
Legendary your armour at your peril !!!

I leave my one handed, two handed, and bow at 100 and perk point them to completion. I only legendary smithing. I use transmute to make gold from all the iron ore. I buy up silver in Markarth every game day and steal it from Dragonsreach. I get 100 gold and silver. Collect enough gems to make jewelry at smithing 15 to get to level 70-80. I have ring of carry weight 37 and smithing 25% increase with a similar necklace. So I get carry weight 74 and smithing 50% increase. That helps get smithing back to 100 quickly. Buy up all the smithing supplies from the smiths and stash them in your houses around Skyrim. This increases your selling of all that jewelry and weapons. I put my gold and silver in Solitude. The rest I put around all my homes. I can then get supplies easier.

I'm slow to level smithing, but I do know of one popular method. Do a few Dwarven ruins and bring back Dwarven metals. Smelt them to ingots then make dozens of Dwarven bows. They increase smithing at a good rate, fetch good coin and the metals are plentiful. I might even give it a go myself. But I hate wasting a perk on Dwarven smithing when I intent to complete the left or Light side of the tree!

All good points - with the 2h I think I would be more likely to start levelling my 1h skill rather than going legend with 2h and continuing. I loves me some Wuthrad though. I'll hang it on the wall and conjure some swords...

I've noticed that after making a skill legendary, it goes back to 15 but seems to level much quicker the second time around so it's not too bad. I've yet to make an offensive skill legendary.

Archery and one-handed take a long time to level - unless you gimp your weapons. In other words, you use a really weak bow or sword so the melee takes longer to complete. When you one-shot your opponent with your Dragonbone bow and dragonbow arrows, the skill won't improve very much because you take so few shots to 'practice'. Switch to a hunting bow and iron arrows and you'll have to hit your target 100 times and you'll level quickly.

The fastest way I've found to level smithing is to make jewelry. If you have transmute, you can generate all the gold and silver you need from iron (which is plentiful). The only hindrance is the precious gems, you'll need a lot of those...

Once you have all the spells, the mage skills level very fast. Alteration-all you need is Telekinesis and you can level to 100 in a matter of minutes. Muffle works really well with Illusion, and Destruction simply destroy atronachs with Expert level spells. Conjuration-soul trap on dead bodies or use bound weapons on summoned creatures. Restoration might be a bit trickier. I've heard using Repel Undead might work to get that up. I know with my magic absorption I might have to drop the Atronach skill to use healing to level restoration again.