I've sunk 400 hours in across the last 2-3 years and 'finished' the campaign many times at this point.
The game is pretty free of bugs, has decent enough storytelling elements and the classes/characters have really enjoyable progression along with great crafting and a hefty amount of legendaries/uniques. If you're looking to completely max a character from 1-100 with mid-good rolls on all slots, you're looking at probably 100s of hours. I'm sure someone can cut that time way down but for average player it'll be a long while. The monolith and arena content with the target farming and etc stays interesting enough given the risk/reward balance you do for your item farms.
Game is closer to grim dawn with the expansions than D3 imo. It's a bit floatier than D2/GD but not enough that I'd consider it worse, just a different feel, depending on the build. Some classes are very mobile and don't need to stop to cast/attack, others can be more stationary. Just depends.
SP characters(ones you create now) will not be compatible with MP, by the way. They won't be allowing stashes or chars from EA and SP unless the updates about trade changed that(I don't think it did). So just keep that in mind.
If you only ever want one character, wait for MP launch.
If you don't mind a SP and MP split, then it's great now, there's plenty to do.
SP characters aren't getting wiped at release I don't think but someone from EHG like /u/ekimarcher can confirm or deny that since I haven't read mention of that bit in a while and not sure where to source.