If you only have a small plot, they are OK. Just not many seeds in a packet. Once they've germinated, anything after that is down to you. The most expensive seed will not do well in poor soil and no light.
I bought most of this years veg seeds last year , when the garden centre reduced all packets to 50p. The only ones I insist on buying new and hence top price are onions and parsnips. The gardening mags give away a lot, and some of the offers in the papers are good, even if you do get inundated with catalogues afterwards.
A couple of good value online companies I can recommend are Moreveg and Simplyseed. Check the quantities in the packs - sometimes they're quite scant, but then you don't always want loads of one variety. They both have a huge choice of interesting varieties, better than you'd get at most Garden Centres. You don't get a picture on the packet, though.
I've bought seeds from Lidl that were fine. I'm sure Aldi would be comparable.
Lidl's great for old faithfuls ie kelvedon Wonder peas. But I agree about the amount you get. But there again how many sprouts/lettuce/toms do you actually need? Some of the varieties are totally different as they come from germany so an adventure can be had at little cost, and I can recommend the Cucumber Seeds from Lidl especially.
With only two of us I find myself with many packets of seed left over even with 2-3yr life! Still, I do have to admit to preferring to pay out for new F1 varieties as feel I need that guarantee of success. I find for a many items it is better to buy young plants/seedlings, although I have unwittingly bought in disease problems from unknown growers ie. Nurseries trading at Markets as opposed to Flower Shows.
Try Park Promotions, they do seed promotions for papers and websites but you can buy from their site quite cheap. They do quite a big range for 99p a pack and sometimes have special promos. Also Kings Seeds are goods, they have two sites, you can buy larger amounts of seeds for a discount on their commercial site or smaller amounts on the gardeners site. They have a wide range and very good quality.
Try googling free seeds
aldi seeds are ok,but i get most of mine from ebay sellers and have had no problems with quality or quantity,look for the ones with good feedback i use four sellers,premier seeds direct is the one i use most.