I've been using the L550 as a secondary phone for the last 3 weeks. My previous WP was a L1020 running WP8.1. For what you pay for, the L550 is okay. I've not had the HW issues described here or elsewhere, yet, so of course the jury remains open on durability. Being a work phone I use it in a 'pocket PC' kind of way; I installed a spare 64GB microSD card I had lying around, and use a Microsoft Universal Mobile keyboard and a Logitech Ultraslimt630 mouse. I occasionally connect a Nokia MD-12 BT speaker for conference calling and for music. I rely heavily on the Office suite (mainly Word), Outlook client for email, and Skype for Business client for instant messaging and conference calls via my corporate enterprise servers.
Frankly speaking I'm quite demanding on my tech. So far the L550 has managed to keep up ` performance is acceptable if not a little laggy sometimes. Based on my particular use-case I would probably be better off with a little more display real estate and much more CPU performance. I also miss the wireless charging and the camera of the L1020. However when you take into consideration the very low price point of the L550 I believe it to be very good value, making up for it's performance and feature shortcomings.
Would I recommend it as a secondary device? Sure, big thumbs up. Would I recommend it as a primary/ sole device? Depending on intentions and budget, then maybe. For me personally, no.