I got hooked on it at the beginning of last year. I was trying it with my 870, but quickly figured out that wasn't going to do. I want to do well, I hate it when I suck at something. I found a really nice used Citori for a good price and bought it. Now we shoot a couple of times a week. Try it, you'll like it.
I bought some extended chokes that I can switch if I need to. I generally use a Skeet and an IC. If it's a longer trap shot, I'll take out the Skeet and put in a Mod. If it's shorter closer in stuff like Rabbits, I might take out the IC and put in a CYL. I'll also switch the barrels back and forth depending on how I'm going to shoot that station. It's almost like golf, you're not going to make a long drive with a 9 iron, or a short chip shot with a driver. If you want just a good all around choke, most of the guys that shoot an automatic use an IC.
When I first started, I was shooting in the teens (out of 50). Within a month or so, I was shooting in the 20's. Now I'm hanging in the 30's and I shot over 40 a couple of times. Yesterday, just playing around and practicing, I think I only missed about 8 or 10 out of 50.
A 20ga is fine. Some guys shoot a 28ga. I shoot a 12ga, so I can't comment of what type of shell to use. A guy at our club puts in a huge order together for us twice a year from RIO. I shoot #8, 1 1/8 oz., 1250 fps. for about $5.25 per box. I'll go through about 4000 rounds this year. Walmart sells Remington 100 round 12ga, #8, 1 1/8 oz., 1200 fps Value packs for about $20.00.
I shoot at a couple of clubs, and the going rate is about $15.00 for 50 birds. So $36.00 for 100 birds ain't bad.
I shot 5 stand last week for the first time. That's like a high speed version of sporting clays and it's fun as schit. It's a little harder because you have to keep your head screwed on tight and pay attention to what you're doing.