The grow bags bought from Lidl have proved ok up to now, the compost is unbelievably black though. To edge my bets and something of an experiement, I mixed it with cumbrian peat and added perlite and growmore when using it in final planting out pots. For potting up I just mixed it with peat and added vermiculite around the stems of seedlings. It's very compact and needs breaking up but I'm not complaining.
It's been used in pots for lillies, gladioli, sunflowers, dalihas to name but a few, all doing really well, dwarf lillies and dwarf sunflowers are in bloom. Also used it in bags for spuds and pots for all manner of vegs both in GH and outdoors. It retains water well, prehaps alittle to well, I've had to reduce watering. I've lost one squash plant after planting out and the odd seedling, giving it the thumbs up.