I am very sure that in my lifetime we will have a definitive answer to one question that has been debated, with little data, for hundreds of years. The answer as to whether or not there is life on Mars will either be a null result if negative, or it will profoundly impact science (and perhaps philosophy and religion) if positive.
As 90's Administrator of NASA Dan Goldin rightly reasoned the biggest possible positive public relations coup for his agency, and therefore for its continued budget, would be if it discovered unambiguous evidence of life somewhere elsewhere in the Universe, besides on Earth.
One of the legacies we see today of that judgment is the almost weekly flow of new planets being discovered orbiting nearby stars. If life does exist outside of our solar system the easy bet is that it exists on planets, so we better find planets to look at for direct evidence of life. We have been able to infer the existence of very large planets by carefully measuring star wobbles, and more recently we have detected smaller planets by measuring their occultations, the way they dim a star as they cross between it and Earth. And just in the last months of 2008 we have our first direct images of planets orbiting other stars.
NASA has an ambitious program using the Hubble and Spitzer space telescopes and the 2016 launch of the Terrestial Planet Finder to get higher and higher resolution images of extra-solar planets and look for tell-tale chemical signatures of large scale biochemical impact of Earth-like life on these planets. If we do indeed discover life elsewhere through these methods it will have an large impact on our views of the life, and will no doubt stimulate much creative thinking which will lead to new science about Earth-life. But it will take a long, long, time to infer many details about the nature of that distant life and the detailed levels of similarities and differences to our own versions of life.
The second of Goldin's legacies is about life much closer to home. NASA has a strong, but somewhat endangered at this moment, direct exploration program for the surface of Mars. We have not yet found direct evidence of life there, but neither have the options for its existence narrowed appreciably. And we are very rapidly learning much more about likely locations for life; again just in the last months of 2008 we have discovered vast water glaciers with just a shallow covering of soil. We have many more exciting places to go look for life on Mars than we will be able to send probes over the next handful of years. If we do discover life on Mars (alive or extinct) one can be sure that there will be a flurry of missions to go and examine the living entities or the remnants in great detail.
There are a range of possible outcomes for what life might look like on Mars, and it may leave ambiguity of whether its creation was a spontaneous event independent of that on Earth or whether there has been cross contamination of our two planets with only one genesis for life.
At one extreme life on Mars could turn out to be DNA-based with exactly the same coding scheme for amino acids that all life on Earth uses. Or it could look like a precursor to Earth life, again sharing a compatible precursor encoding, perhaps an RNA-based life form, or even an PNA-based (Peptide Nucleic Acid) form. Any of these outcomes would help us immensely in our understanding of the development of life from non-life, whether it happened on Mars or Earth.
Another set of possibilities for what we might discover would be one of these same forms with a different or incompatible encoding for amino acids. That would be a far more radical outcome. It would tell us two things. Life arose twice, spontaneously and separately, on two adjacent planets in one particular little solar system. The Universe must in that case be absolutely teeming with life. But more than that it would say that the space of possible life biochemistries is probably rather narrow, so we will immediately know a lot about all those other life forms out there. And it will inform us about the probable spaces that we should be searching in our synthetic biology efforts to build new life forms.
The most mind expanding outcome would be if life on Mars is not at all based on a genetic coding scheme of long chains of purine bases that decode in triples to select an amino acid to be tacked on to a protein under construction. This would revolutionize our understanding of the possibilities for biology. It would provide us with a completely different form to study. It would open the possibilities for what must be invariant in biology and what can be manipulated and engineered. It would completely change our understanding of ourselves and our Universe.