I tried one once, and returned it right away. I found it... uncomfortable. I didn't like the feeling of extra weight up that high. It made me feel less stable, especially when I was just taking off from a standstill. If it's a pack up there, it doesn't worry me much - if I dropped the bike, I'd only skin my own knee. But, I didn't like that Talia was up so high in the event there was an accident, and given that I felt less stable than usual it made that discomfort even worse.
Also, it took up the panier rack and the rear mounts, so I couldn't carry any rear panniers, leaving me with only front panniers for things like groceries. Seriously reduced my cargo capacity. (Well, I guess I could have pulled a cargo trailer.)
I ended up with a Burley trailer, and we *love*
it. She's down low, she can doze off comfortably, and even if I drop my bike I can't overturn the thing. I can fit groceries in the back of it. It leaves my rear mounts free so I could use my pannier rack at full capacity too, if I wished. And, the guy at REI who exchanged the seat for the trailer for me said that, while he'd never heard any stats, he's heard annecdotally from several customers that car drivers seem to be more respectful of trailers than seats. He conjectured that was because they are much more visible.
I think I'd be more comfortable with a front seat instead of a back one, but at the time I couldn't find one. But, with kidlet two on the way (wah! I can't bike this summer!) I may end up trying one. I have a single-seat trailer, and even if I buy a double-seater, I don't think Talia could sit there comfortably with baby in car seat back there. So I'll have to put Talia in a bike seat and baby in car seat in the trailer... Eventually we'll get a double-wide, but not until kidlet numero dos can sit up reliably so we don't have to use the car seat (next summer, hopefully.) Sorry for rambling... did that make any sense?