Well, I can tell you, I tried drawing with just a mouse before... it didn't end well...nope.... not well at all. It literally looked like I went back to preschool with art.
And I would too recommend a tablet, just like
@Pepper-Head suggests. Yes, a tablet is a tool, but quite a worthwhile one. It does take a little bit of getting used to, but I love mine to death and don't know what I'd do without it. I own a wacom, intous medium tablet, which I think was maybe like $80 ish at the time. Its a large enough style that gives you plenty of hand space for movement but not too big that it swallows up your desktop. Almost about the size of a standard notebook really.
The tablet lets me draw the way I want to draw and use angels that are best for me. I did buy some of the good plastic sheets you'd normally use for paper or projects in binders. I put my tablet in it. I cut off the part where the binder rings go through and tape the other end. This protects the tablet surface and my pen tip so I don't go through tips fast or damage the surface of the tablet as terribly.
Tablet will not improve your artwork. Like anything else, you have to practice with it and keep trying to figure out what techniques work best for you. Drawing digitally does have its perks though since you can easily erase mistakes and bend lines better to what you'd like. I use it with a combo of Manga Studio 5 and occasionally other programs. Each program will have things set up differently. I personally enjoyed working with Photoshop CS6 but currently still do not have the money to own one, so since I got Manga Studio as a gift, I use that to the fullest and have learned quite a bit about it. Like any tool, once you've practiced well enough, the tablet becomes quite the ally to an artist. I've only ever owned 2 tablets in my life and I love them both. The first one finally had to be retired but if you take care of it, it'll do good for you.
Like I said before though, the tablet is just a different way of doing things but the skills are still dependent on you. Yes, there are ways to make things faster but you still have to have the knowledge to know how to use and do it right so it doesn't look like a mess. Over the years, I've improved as a traditional artists and ultimately that shows in my digital artwork too. To me, a tablet is well worth it for an artist who wants to partake in the digital medium on a higher level than pixel art.