Bit of recent chat on FB about this. Copied some of it below. My take is that they are not as reliable, complex to pack, more benefit in the mountains, might need to consider a back up round or square.
Post by RC - Following on from my woopsi on August Bank Holiday Monday and my reserve parachute deployment over the edge of Sheffield. I have decided to get a stearable reserve. They seam to be very popular in central europe. I have decided to get #highadventure Beamer 3 reserve. Mainly because there promotinal youtube vidios all show them working with the main glider still attached. Ie real life flying. They deploy with the breaks locked down. So behave like a round parachute untill you release the breaks and stear to were you want to go. Ill hopefully never have to use it but should the worst happen again, at least I can stear to a safe landing spot next time instead of takeing pot luck on my landing spot.
TB - A lot of pilots avoid them because they're more expensive, heavier and viewed by some as more complex to repack, even though they're not. They're just very different from a PDA.
JB - Yep, agree with Kirsty Cameron, overall Beamers can create more problems than they solve, as well as a longer deployment time (I believe). If you do have a beamer it is best to be as part of a 2 reserve option: one round or square to be deployed in anger, low to the ground or terrain and the beamer to be used with more altitude to spare giving more deployment time, and getting rid of the main and gaining control of the beamer
ML - .....diamond cross square steerable......I think it's the best thing on the market at the moment.
This looks interesting - https://youtu.be/F8cEEOoJJ3c