We have been using plenty markets ERP system now for 6 months after switching over from Linnworks. For us the main reason to make the switch was the lack of certain functions in Linnworks like the ability to have tiered prices for both B2C and B2B customers. Plenty markets provides this plus the ability to host a dedicated B2B frontend website as part of the whole system. Effectively we got a complete B2B website for free! Our main B2C store is a Magento website and we use Plenty Markets to manage our inventory across all our sales channels (Magento, Amazon and B2B sales).
We were one of the first customer in the UK to be connected via Magento, so initially there a big setup to go through. Now however the Magento connector should be a seamless process.
One other issue we encountered was the ability to assign a shipping service based on post code, as DPD charge different prices for the Highlands, etc. Plenty Markets recognise this as something important to implement and I believe this feature will soon be available.
Overall the system is quite comprehensive and has a lot of powerful features in the backend that are only available in other ERP systems that will cost you 2 or 3 times more at least.
In setting up the system initially we received a lot of one-to-one support, and our requirements were quite demanding. Once the system is setup it has been quite smooth and training users in-house to use the system was not so difficult.
So overall I would rate Plenty Markets 5 stars considering the value for money against the numerous built-in features and reliability.