I was in two-weeks ago and ordered tea because I wasn't feeling so hot. The barista recommended an excellent tea and the atmosphere was conducive to conversation. Note to other shops: I may quibble with their coffee, but I've always received excellent customer service at Mars.
Add in that they have wi-fi and comfortable booths, I'll probably be in more now that I'm a grad student starting this fall. The place has improved since my initial review.
Mars Cafe is an excellent place to get caffeinated. Alongside their fantastic coffee and espresso beverages made with Kickapoo coffee, Mars carries a selection of teas from Gong Fu and makes a delicious yerba mate lemonade. The employees are super friendly, the bathrooms are clean, and the wifi is decent. I've never had anything to eat here, but they have a selection of salads, pastries, and sandwiches.
This place also has my (somewhat worrying) #1 favorite characteristic of Iowa coffee shops - beer and wine! Basically they just have a house red and a house white, but they change up their beer selection and there's usually something interesting.
One unfortunate downside - for a college cafe, they could definitely stand to extend their hours. 10 pm isn't really that late to begin with (or maybe it is for Des Moines?), but they seem to have altered (shorter) hours very time I come in...