Fountain pens are very reliable, as long as they're made in the last 20-30 years and cost more than $30-40. There are older pens that are also very reliable, but because they were made differently you'd need to know a little about pens to find one of the good ones.
The ink is almost as thin as water and flows down by gravity and capillary action to the nib; the only times a fountain pen won't write are when it's ultrawindy and dry, drying out the ink so fast that it can't flow down, and upside down. I've never had the windy thing happen, but there have certainly been times I wanted to write upside down, but ultimately I'm not an astronaut, so I just lowered the piece of paper.
A good fountain pen starts writing the second it touches paper, and writes smoothly until it runs out of ink. Leaking is also a thing of the past, except with crappy pens that you might buy at Walgreens, for example.
Good brands that don't cost a fortune are Lamy, Parker, Waterman, Faber Castell, and Visconti. I may have forgotten some, but all ye needeth to do is ask.