Has anyone tried these? I'm looking to build a longbow for indoor league and field archery.

I assume these kits will become good bows if I do my part.

Recommendations?

Thanks.

any one of them, most of the hard work is done, You just need to tiller the bow to your weight & draw then put a finish on them. Great way to start making bows...

The bow I want to build is for my girlfriend. She wants to get into trad archery, and building a bow from a kit is something we could do together.

I've built longbows from scratch 20 years ago with my high school art teacher. I could probably do it again, but the kit would save me time.

Take a look at the offering from Dryad Bows-they are a sponsor too and the video that comes with the stave is very helpful.
Dryad's tillering method is quite idiot proof-even worked for me!

I second the dryad recommendation. I made one and
it shoots well and I get lots of compliments about it. People are amazed I made it.

How long will an all-natural bow last? will the wood lose it's spring even if it's well cared for?

A bowyer once told me that all bows have a finite number of shots in them.  Some, naturally, will be much longer.  There may be more factors, but I'm sure many of the major ones would be type of wood, design, glue and care.  I wouldn't worry too much about it, just take care of them and shoot them.

 I also second the Dryad bows.