With the right character, delve is great fun and a quick way to level without too much risk. My best guy can run at the 200 depth without much trouble. If I go deeper I have to pay much closer attention to what I'm doing. For Delve you have to be able to run and kill as you go and deal with large mobs in a storm of screen action; phasing is a huge help since you don't get blocked at the narrow pathways and you get hit less. I really like phasing in general and try to add it to all my guys."Endgame" is a misnomer. There's a whole second campaign after you kill Kitava, The Atlas of Worlds. This is what people are referring to when they use the term "maps." I'm not sure anyone calls it the "endgame." In fact, I'd say the opposite is more common: a lot of people consider the game to only really begin after they blaze through Acts I-X as quickly as they can. The recent DLC completely revamped the Atlas of Worlds campaign, too.
And if you enjoy the Delve mines, that's basically a whole third campaign. I think the furthest I've ever gone is like depth 40? I know there's stuff down there that I've never seen.
I think my highest-level character isn't even level 75. I haven't done many of the maps because I'm not great at getting through the Acts quickly before the next League starts. I think at some point, I'm just going to have to skip one of the new Leagues and play one of my existing characters. I might take up the Necromancer I was playing in Blight and see how she does in the maps.
Speaking of which, I know your existing characters get a full Passive Skills respec with each League. Does anyone know whether you have to use that right away? Can you play a few maps, just to see where the character's weaknesses are, and then use the full respec? I think I have 4 characters around lvl 70-73 who finished the Acts, but it's been a while since I've played them and I wouldn't know how to rearrange their Passives without playing them first.
The full respec can be used anytime. I have guys where I took the respec and haven't used it yet; so they are level 85 with 100+ points to spend. I also have guys that were built three leagues ago and still have a respec waiting. And yes you can use a character from an old league all you want without respecting and then respec it later. You just can't change the passive tree at all or you loose the respec until the next league. If you want to try out a standard guy's build in maps, I suggest paring up with a strong build and run a dozen maps. Not only will you level quickly, you can avoid dying and complete your atlas. Then you can respec your guy based on what you learn. With a strong partner, you can go slow and try stuff and have a back up when the boss or mobs are overwhelming. Once they fix the memory leak, I'm available. right now I can only play about 20 minutes before I have to quit the game and begin again.
Also, getting 20/20 gems is a huge buff. That is hard to do if you only have level 70 characters (unless you have lots of currency to spend.)