With a second hand value that's roughly three times its original price, the Roland TB-303 is perhaps the single most overrated synthesizer of all time. Sure, it's good, but it's not that good. It can produce a fantastic sound, but people's perception of it seems slightly inconsistent with what it can actually do.
Don't get me wrong, it can create a smooth, pleasant bassline, a crazy, screaming acidline and anything in between... but that's it. It's one of the least versatile synthesizers there is.
First, let's look at How you play it. As the TB-303 predates MIDI, there is no way to get another instrument to tell it which notes to play. The only way to get a sound out of the 303 is to use its built-in step sequencer. As is the case with Roland's similarly overrated TR-808 and TR-909 drum machines, this involves punching up to sixteen notes or rests into each of its patterns. The "slide" button offers portamento for any given note, while "accent" (also featured on the 808 and 909) allows notes to be emphasized, but while these are welcome additions, the sequencer still offers little variety.
Any music based around a 303 will have to conform to this simplistic step sequencing, most likely with very few chord progressions. This is due to the awkward nature of transposing or changing the pattern being played, which has to be done in real time. The inevitable nature of music composed via step sequencing is probably at least partly responsible for electronic music's reputation of being repetitive, something that needn't be the case.
This problem has been overcome by several of the synth's imitators. Freeing the musician from having to use a studio that can support the now ancient Sync24 standard which is required to synchronise the TB-303 to modern hardware (superseded by MIDI in the mid eighties), and from having to think in terms of patterns, more complex basslines and acidlines can now be programmed into these similar sounding devices. This arguably makes some of the TB-303's so-called clones superior to the original machine.
The other limitation, although not as severe as the sequencer, is the 303's sound. It is monophonic and monotimbral, and it has only two periodic waveforms to choose from (a sawtooth wave, and something Roland claim to be a square wave but is actually something else entirely). The selected waveform is then passed through a resonant lowpass filter with a decaying cutoff point. It is this filter which gives the 303 its distinctive sound. While the sound itself is great, this is no longer the advantage it once was, as it has been used in so many songs that it has become a cliché.
While it is now possible to emulate the TB-303 using software or relatively modern hardware, you should still think about whether you actually want to. If standing out from the crowd is something you want to achieve with your music, it might be worth exploring a less trodden path. If, on the other hand, you just want to write something that is easy to dance to, go ahead and put the 303 or one of its clones at the front of your mixes.
Personally, I find the machine's sounds to work best at the back of a mix, adding an extra element to a song but not drawing too much attention to itself. A good example of this is Fatboy Slim's chart hit Praise You, which features a TB-303 acidline buried behind a piano sample, vocal sample, some drums and a bassline near the end of the song (pretty good self-restraint for someone who titled one of his tracks "Everybody Needs a 303"). Even then, it's probably best to avoid putting it in the majority of your songs.
For a good example of songs centered around the 303, you should check out Luke Vibert's album Lover's Acid. If you still think you can make something original with this machine, go ahead. It would certainly be an achievement to be proud of.