I have been to Siem Reap....Spent 4 nights at the Ankor Village and did some touring....Knowing what I know now I would do the power tour thing. On the first flight and back on the last via a tour group and I HATE tour groups.
Spend alot of time doing transit, maby 2 days in time out of 4 days there. Siem Reap is a dusty place in the old town. Out near the airport is a newer section but no culture...there has been alot fo money from Japan to mnake it a tourist place etc. The tours are very oranized and time efficient. I'm very comfortable in strange places in Asia and found Siem Reap to be boring and the Temple to be a bit repetative....Too much talking etc.
If you have lots of time and like to waste money go for a few days....I'm glad I went but would rather have done more efficient thing. Cambodai is years behind thailand and I would have thought it were many many years ago except for the MOBILE is everywhere.