growing from clippings vs seed (fruit trees)

is it worth while growing from seed ever? or should i switch to growing from clippings?

Plums, cherries - probably
Apples, pears - no, like "magicbastarder" said, they'll basically be inedible mutts in most cases. Sometimes you get a nice new variety though, many fruit varieties popular today actually grew from chance seedlings and discarded apple cores.
The chances aren't great for getting a nice variety though, most are bitter and fit only for animals. You're more likely to get a cooker than a dessert (eating) apple. Pears are the same.

Growing from clippings is called "grafting" where you get a branch or bud from a variety you like and merge it with the roots and stem of another tree (the rootstock.
Rootstocks can be the same species of tree or a related one. Pears for instance are grown on Quince rootstocks very often (a pear-shaped fruit related to pears and apples that we don't really grow here but whose roots cause trees to grow smaller than pear roots would). Pears can also be grown on Hawthorn, but I'm not sure of the details.

if growing from clippings, how is the best way to do this?

I don't know, I've never tried it myself. You can read about it here:
http://en.wikipedia.org/wiki/Grafting

One interesting concept is a "family tree" where you graft two different varieties of apple onto one rootstock. This enables you not only to get two kinds of apple, but also allows them to act as pollination partners for each other (you'd have to make sure they bloom at the same time though).
It looks complicated though, so I'd buy one. An interesting concept though.

Remember that grafts are a weak point, so choose a grafting technique that will give a stronger bond.

are the aldi/lidi trees any good?

I think Aldi, Lidl and Poundstretcher (and maybe Tesco) source from the same supplier.
Certainly the Cherry trees I've seen at Aldi had different images on the package, but the description was exactly the same as the one on the packaging of a cherry tree I bought from Poundstretcher last year.

I recently put in an apple and pear tree from Aldi and they look to be OK. You don't realise how small they are until they're in the ground though, but they'll grow.
The apple tree's buds already look like they're about to come out soon, but the pear's don't so far, so I'm not sure about that.

The pear tree is certainly better looking at the moment though, the trees on display got knocked about a bit and the apple was one of them.
Its also double grafted (sometimes they're not compatible with the rootstock, so they have to be grafted onto something else and then that in turn is grafted onto the intended rootstock - something complicated worth considering).

They look nice enough and I expect them to grow decently enough. Go for it. They've sold out here though, but Tesco have some (more expensive, almost sold out too).

so it is unlikely that i will end up with a great new variety? ... i'll stop planting seeds so

Well the chances of a good offspring aren't so high, but you may get lucky. Consider though that you'll have to raise the tree for 5 years until it fruits to find out. Imagine after 5 years or so going and eating the fruit only to find out that it was disgusting - I'd be pretty cross!
Then again, you could always just grow them ornamentally. Fruit trees blossom remember, so even if fruit is rubbish they can look good in the garden. From seed they'll grow taller though, so'll have to be cut back occasionally.

When people talk about apple and pear trees being hardy or not a lot of the time it is very fruit-focused. The actual trees themselves should do well in most situations in Britain or Ireland even if the blossom and fruit can't cope with the weather.

The fruits will most likely be like crabs or be more like a cooking apple if grown from seed.

Sourcing good quailty rootstocks and scion wood is key and IMO Lidl/Aldi is not it. Making sure the variety will actually grow in your area is very important for long term success.

They provide grated trees, not rootstock alone. I'd find it rather futile, buying a fruit tree which can produce nice fruit only to cut off the grafted plant and nick the rootstock.