For entry-level synths, there are two models that come to mind:
The microKORG is pretty rad. It has a spacious layout for its size and has a built-in vocodor (which can be realllly fun). the biggest downside is the tiny, cheap feeling keyboard. I would've been sold on this model but in my opinion, full size keys are a must. It has a warmer sound than the Alesis models, from what I'm told. I haven't had a chance to play them side by side, so I don't know how valid that is.
The Alesis Micron is what I use. Full size keys, sequencer, vocodor (but doesn't come with a mic). I really like the workflow of the micron's main parts (rhythms, patterns, programs, setups), but one of the main flaws to the micron is how there are minimal buttons. Everything you program is controlled with one knob and is displayed on a tiny ass screen...this is especially difficult if it's your first synth. It is my first synth, and this layout makes the learning curve steeper than that of a microKorg, but I honestly feel like the Micron is a higher-quality synth. Another flaw with the Micron is that its sequencer is based on 4/4 time. Not too big of a problem, but for compositional diversity, it can be a pain having all your songs in 4/4. Generally you'll be arranging on an external sequencer anyway, so I wouldn't invest too much focus towards the sequencing function in low-end synthesizers.
Another model to look for is the Korg R3, which I hear has higher specifications than the Alesis Micron and costs only a little bit more. I have only used this one minimally, but it produced some really nice sounds.
Be sure to check out the Harmony Central boards too. It's filled with all sorts of synthesis freaks and industry types.
http://acapella.harmony-central.com/forumdisplay.php?f=18&order=desc
