As already mentioned, story points are a relative measure of complexity. One can use power of 2 series (1,2,4,8,16...) or a Fibonacci scale (1,2,3,5,8,13,20...) for estimation. As espoused developers are quite adept at saying something like this:
Feature A is almost twice as hard as Feature B
But it's really difficult to say 'how long' will this feature take for implementation. You let that be balanced by velocity. So if something was estimated as a 5 but turned out to be a 13, a slower velocity would normalize that for the iteration (or you could re-estimate).
Now, there is another alternative, it's called 'ideal days' (some what similar to man-days but I'm not sure if that's what you meant) and I know of quite a few teams who prefer that. Ideal days are to be interpreted as:
If that's all what I do after coming to office and take only the necessary breaks, have no interruptions and will have everything I need to 'implement the story' i.e. no peripheral activities like meetings, responding to mails etc.,
Mike Cohn, one of the many well know agile evangelists provides the following comparison between story points and ideal days
- Helps drive cross-functional behavior i.e. teams estimate stories w.r.t. total implementation complexity all the way from UI to DB and back.
- SP estimates don't decay i.e. a few months from now a 5 point story is still likely to be 5 points, but an ideal day estimate may change depending on the acquired development skill/speed of that particular programmer
- SP are a pure measure of size i.e. they only and only reflect size w.r.t. complexity. Period. No duration etc., thrown it. That's the job of velocity. But not so with ideal days. In fact with ideal days there is a tendency to muddle it with calendar days. Keeping it abstract as SPs fights the temptation to compare with reality. Just a measure of size. No nonsense.
- Is typically faster than ideal days. It may be tricky for the first couple of stories, but once you get the hang of it, it's faster.
- Different developers can have a different take on their ideal day estimate for completing a story. I could do the same in 3 and you could in 5. SPs are more or less uniform across the board. They level the playing field so to speak.
- Easier to explain outside the team; for obvious reasons :)
- Easier to estimate at first as mentioned above. But once you get the hang of SPs it comes naturally
Now, which one to choose is up to the team. However, as most answers here and my personal experience, I prefer story points. Ideal days don't really have that much of a benefit over SPs (and Mike Cohn also advocates SP along with many other agile evangelists).