I've used them and many many other suppliers. In my view, as with most things in our hobby, "you buy cheap you buy twice".
Reasons to buy good stock.....
Argument 1:
Take Glosso for instance, from these Asian suppliers it comes as a stringy, unplantable mess for £2.50, a pot of Tropica Glosso from TGM for instance costs £5.00 but I garantee that once planted you'd cover twice as much substrate.
Argument 2:
A Plants Alive pot looks to contain about 20 plantlets that have been grown submersed and have become very leggy, which makes prepping a nightmare - the pots from Tropica and Dennerle contain about 50 plantlets and can be pinched directly from the pot and planted into the substrate with minimal effort. I've seen AquaFleur selling pots of both types, long and leggy and tight and green.
Argument 3:
You may think that a plant that's been grown under water would adapt quicker to it's new environment than one grown above the water, this is not the case. Plants grown above the water in Europe are extremely strong and store lots of energy. They are then packed in boxes and shipped the short journey over here, generally overnight. The shops and webstores that stock these plants often order them in specifically for the customer which generally means they spend very little time in their holding tanks which means the customer gets a seriously fresh pot of plants which are packed full of energy. Also, the shops ad webstores that stock decent plants tend to treat them well. The plants that are sent in from Asia are grown outdoors (which is one of the reasons for the need to use pesticides) and are then placed in a bag and then flattened in an over-filled box (they often pay shipping by the box, not the weight) for up to a week. The UK supplier then unpacks the box, sorts your plants and ships them to you. (In my experience the word sort certainly didn't mean check, 70% of the order was wrong and the response when I wanted it rectifying was two weeks of ignored phone calls.) So these plants have been squashed and in the pitch black, getting hot and cold for a minimum of 5 days and a maximum of 10 days before they get to you. Is that a good way to start your planted tank?
Argument 4:
I like to support those who support our hobby. If you look at all the aquascaping competitions around Europe, they're all sponsored by AquaFleur, Dennerle or Tropica. Yes they get good bang for their buck but without the help of these guys we'd all be stuck with what is in my view inferior plants.
Based on my experiences with Plants Alive I won't buy from them again.
It's not necessarily a case of them failing but you ask any of our "top aquascapers" where they get their plants and I bet I know the answer