The Marangu route is the "tourist" or "Coca Cola" route, and, indeed, you can buy soft drinks and bottled water all the way to the Kibo hut! The last 4000' to the summit from Kibo is very tough, but my guess is that about 2/3 make it (with the extra day to acclimatize), maybe higher if conditions are perfect, as they were when I went.
I took Diamox (Acetazolamide), and heartily recommend it. My companions were both at least as strong as I, but each had some altitude sickness; they did not take Diamox. I had no bad effects from it, though it is a powerful diuretic, and the water has to be replaced. I estimate that perhaps 10% of the climbers on the Marangu route were taking Diamox, and yet the majority made it (at least to Gillman Pt.; the attrition from there may have been much greater), so....On the other hand, I met an American girl who said that in her group, those on it made it, the others didn't.
I strongly recommend taking an extra day to acclimatize at Horombo. The success rate is substantially higher among those who do.
I used Zara International of Moshi, Tanzania, and heartily recommend them. Our guide was very good. Zara's e-mail address is zara@form-net.com.