1. Are they messy
Not all the time, but every FP user eventually has a leaker that results in a stained shirt pocket.
2. How do they wright (smooth, clumpy, etc)
FPs are much smoother than ball points. Roller balls are somewhere between. But a well-tuned FP makes writing a pleasure. More importantly, the lines laid down by FPs are crisp and distinct, and just look more elegant than BPs, RBs, or gel pens. And the selection of inks is almost unlimited, something that cannot be said for other kinds of pens. (Although one manufacturer does offer a roller ball that can be refilled with FP ink.)
Be aware that the writing characteristics of FPs is very much related to the paper being written on. Some papers (eg, Clairfontaine) accept FP very well. Others are not so friendly. One of the more common tests is how well FPs write on cheap duplicator papers. And also, different inks have different writing characteristics.
3. What maintenance issues are there
Maintenance is a factor. You have to refill the pen. The simplest option is the cartridge - that's closest to the convenience of ball points. But there are other filling options that require more maintenance. And filling can get messy - I use rubber gloves when I fill my pens to avoid inky fingers. Periodically (every few months, or every few refills) you need to flush out the pen. If the pen is to be left idle for weeks-months, it's wise to empty the ink and flush out the pen. FP nibs need to be 'tuned' - which is a term for setting the tines to get optimum ink flow and smoothest writing. Experienced FP users can do their own tuning, but there are 'nibmeisters' who offer that service. Most FP kits used by pen turners use steel nibs that generally will hold their tuning for a very long time, but it is also possible to get gold nibs that need to be tuned more frequently. And experienced FP users know that you never want to loan an FP to someone who isn't familiar with them - loaning a pen can accelerate the need to have the pen tuned.
4. Any other Fountain Pen Basics ?
FPs are a niche. They appeal to people who want to have a little elegance in their lives to offset the noise and clutter of computers, TV, modern politics, and the other things that tend to elevate blood pressure. In general, they are a bit more expensive than other kinds of writing instruments. But those of us who use them wouldn't be satisfied with anything less.
The other point is that switching to FPs is not a single point decision - FPs are more of a life style issue. If you want to use FPs, then you have to be prepared to deal with the maintenance, with filling, with occasional messiness, with the myriad choice of inks (colors and characteristics), etc. If you have read this site frequently, you have no doubt noted that the 'simple' matter of pen finishes is something that people have strong, often differing opinions about, and there are millions of options and choices to choose from. The subject of FPs is very similar. If all you want is something to sign credit card slips, then a Bic will suffice and will only cost 12 cents. But if you want something that will become an overwhelming passion, the FPs will do it for you.
