I don't have experience of the JM home fragrances, but for the personal fragrances: I do find most of them are weak in terms of longevity, even for cologne. I suggest testing/sampling and giving anything a full trial (wear it for a day) before buying a bottle, because they are relatively expensive.
A big part of JM's appeal is that there's a large variety (lots are limited edition) and that many of the scents are (or were - there's a lot going on in smaller houses right now) kind of adventurous and unusual, and highlight English botanicals. I'll also say that longevity varies: one of my favourites from the line is Pomegranate Noir, and it IS tenacious! I also found Orange Bitter (kind of like a greenish herbal Campari) very long lasting, but I think that was a limited edition.
Jo Malone also has a line called "Jo Loves", which gives her a bit more control now that Jo Malone itself has been sold to a huge multinational. Not sure if this info is current, but for a while Jo Loves shops in the UK were giving out 2 generous (about 2 ml spray) carded samples for anyone who stopped in and asked. (My favourite is White Rose & Lemon Leaves). Space NK also carry the line and will give samples.
... there is definitely nicer out there and for a lot less ££... If you have time, I'd love to hear about other options you (including anyone reading) recommend! Here's a sampling of mine:
April Aromatics. Germany-based. All kinds of botanicals from around the world, but quite a lot European. Expensive, though!!
Cloon Keen. Irish perfumes and candles from Galway, with a really wide international distribution network. My favourite of theirs is Lunasagh: another herby fresh white floral citrus.
Diptyque. I used to love them like crazy; distilled scent of the Greek/Balkan countryside. Their prices have escalated and they've discontinued their most "natural" scents, but still worth a look.
Goldfield & Banks. Australia-based so you won't necessarily find specific English floral scents, but worth a sniff if you just like nice long-lasting natural scents.
Kingdom Scotland. Portal is my favourite. They only have four scents, but I'm waiting to see what they come out with next. They're carried by but otherwise you can order a sample pack or travel set online.