Well, yes, they work.

In single player, you'd have no issue at all using them. And although they're still quite useful in Multiplayer, they suffer from several issues.

Minecarts don't really suffer much directly, but depending on how fast you're going, and how many people at once are using Minecarts, the server can get quite stressed. There's also really not much of a need for Minecarts, because you'd only use them to get from one set destination to another, and many servers have /home and /sethome, /spawn, and survival faction servers have /f home. Some servers even have /tpa. Why would you go to the extent of crafting rails, and powered rails, or powered carts, when you can just teleport?

Boats would clearly be more useful due to them not requiring a set path, but they are plagued with another serious issue- the lag in controls. After relaying a control to your boat, sometimes you have to wait a few seconds for the boat to respond. This can make boats extremely difficult to control, and almost all the time you'll crash your boat into the shore, or have to leave it quite a distance from the shore in that it's a hassle to get out and into it.

There's also the issue of moving or destroying these two things, you might try to nudge your boat and a few seconds later it flings off away from you extremely fast.

They can be useful, but to me, the downsides are just too annoying. On multiplayer.