I needed something lightweight, yet well-enough-cushioned, for road racing and tempo training.
I chose Brooks Racer ST 3. I have never had them before, so the following is all I can say about them now.
First, they are light. My pair (US size 10.5) weighs only 520 grams (18.3 oz). Compared to my trail running shoes, they feel almost like running barefoot.
Second, I figured that if Scott Jurek can win both Spartathlon and Badwater TWICE with these shoes, they can't be that bad for ultramarathons. However, officially they are recommended for distances up to marathon only. I guess at the end of the day everything depends on your weight and running style. They should be ok for me though, as I'm not going to attempt anything like Jurek in terms of speed or distance, and I believe he is heavier than me (I don't ever allow myself to get above 69 kg = 152 lb, but I'm often leaner).
Third, they were not as expensive as most other shoes. The price has to be right because I have to buy new shoes every 6-8 weeks. Generally the prices in Europe, and especially here in Finland, are extremely high, so it's worth all the trouble to find bargains.