I used Minimed for nearly 10 years, and switched over to the T-slim almost 9 months ago. While I wasn't not happy with MM for a handful of reasons, I don't know that I much recommend the T-slim either....
In random order, here's some of my thoughts:
Tandem doesn't sell/distribute either pumps or supplies, everything has to go through a distributor, which in my experience has been kind of a pain.
I HATE HATE HATE the loading process, it takes on average 20 minutes, as opposed to the 5 minutes with MM.
The menu system is awesome; its 1000 times easier to see and adjust current settings!
The touchscreen feature is neither here nor there, it works just fine but I didn't see there was an issue in having actual buttons to push.
The battery took some getting used to, but has a million recharging options so it hasn't become an issue at all. I think it lasts me almost 8 days between charges, and I have a couple external battery options that work fine too in addition to the standard wall plug and USB options.
I haven't tested the watertight rating of it, and probably wont until summer again, but that was one of the key reasons I finally ditched MM. (I had the nerve to run a marathon on a rainy day and the rain killed my pump; that was the end of MM for me.) The battery charging door seems a little... weak... though to be the key to keeping it safe when submerged or even just rained on.
The Tandem customer support people are very friendly, but its still a young company and so they don't in general have the years of experience that MM had. I assume they've gotten or will get better, but the first few months I felt like I was calling them and reporting a problem and then calling back a week later to report the work-around because I could find it first.
Maybe not an issue in a big city, but I lost my community support connections when I switched pumps, I no longer know 6 other people who I can call when a shipment goes awry to borrow supplies from.
Because Tandem is so new, there aren't many "certified trainers" for it. Additionally, when they sent me mine they didn't even pretend to try to set up training, they offered the user manual and the website, and said more or less that they'd help me figure out how to set it up if I called the tech support line, but there was exactly ZERO formal training or follow-up.
I guess its a pump, it puts insulin in as best as I can tell (although I do wonder sometimes?), it does the math for me, but it doesn't seem to do anything spectacularly different than any other pump. If I had a chance to do it again, I'd wait until their second generation came out just because then they'd have more experience and I think some of the things I had to work through in the beginning wouldn't come up for the second round....