While mp can be fun (or frustrating depending upon whose going to connect), the SP for me has been a great time.
For someone who had never played DOTA, at first I found the normal AI to be very hard. It felt like an accomplishment the first time I finally beat a team. Playing all the maps and then adding 1 Hard AI to the mix, I was able to finally beat it, then 2 Hard's and then my friend and myself were doing 2v4 Hard.
This past weekend, I have graduated to Nightmare. I can now handle myself + 2 hard AI's on my team with 3 Nightmare's on the other team. It's an intense back-n-forth battle, especially now that I'm having to help out my AI brethren.
The thing is, with the number of DG's, I just randomly assign DG's to my team and the other team, and combined with the maps, this gives me quite a number of variations to play and keep me happy. Not to mention, I read up on ideas for different DG's and strategies and I try those variations out.
Not everyone is a MP god. People have complained about no campaign, or SP, but I'm here to tell you SP is alive and well and can keep you entertained for hours on end.
The game is gorgeous, the gameplay is what you make of it. You don't need MP to get your entertainment value.