Alternate realities are hit and miss with me. It's its in graphic novel form where that universe will have an ending as soon as the book ends? Yeah, I love those! Gives you a break from the status quo and yet doesn't over complicate itself by trying to make itself a big open ended universe.
Alternate realities such as Ultimate Marvel? Well I only really read Ultimate Spider-Man up until the death of Ult. Peter Parker(How I hate that I have to differentiate now so draw from that what you will. Now I do like some of the ideas in the Ultimate universe, don't get me wrong. Mutations being due to a virus instead of genetics is interesting and gives more credit to the "hated and feared" thing mutants deal with. Captain America being a confirmed superhuman instead of the "Eh, KINDA superhuman" he is in the mainstream 616. However I don't like Ultimate Marvel as a whole by ANY means.
Alternate realities I don't like are the New 52 DC Universe anything past the original Dark Knight Returns since Frank Miller's Dark Knight Strikes Again just further shows the man's increasing insanity.