Mostly been said , but my 2 penneth is Full Features / Limited Time but not too limited .
As a Red Gate site , I'll use SQL prompt as the example , I used up my 2 weeks freebie , I was so impressed I bought ( I already had the Prompt SQL Predecessor) . Months later I was still finding new bits , maybe I'm slow. If you were prevaricating as to whether to purchase , it just have tipped the balance in not having enough time to evaluate properly.
2 Weeks is a bit short especialy if the investment is high, a month is usually sufficient.
Another niggle is being forced to use some one else's "Tame Database" rather than applying the product to one of those "Real Db with warts" that none of us have (!).
Most vendors will extend trial licenses but they hesitate at more than a few days to avoid over trialling . I have also come across some , Codesmith for example, who will allow "Tame Db" restrictions to be lifted to see how it really performs but for a shorter period.
I know there is the commercial aspect to consider but I believe speculate to accumulate