As with everything, there are pros and cons, so I'll try to cover everything for you in this post.
THE DOWNSIDE - READ THIS FIRST
Before you start, you must realise that if you have a special background or header image (which you may have commissioned, or it may be a freebie) YOU WILL LOSE IT. It can't be helped: the code will override any previous background and header images.
If you have a special header, I'd advise you to copy it to Picasa, Flickr, Photobucket etc, so you will be able to reload it from there. Unfortunately, this won't help re backgrounds. They will be GONE. Then again, if you are reading this, you are obviously thinking about a new look anyway!
You will be asked if you want to save your widgets, which means you can - but I would still advise that you check previous articles on how to back them up ... just in case.
If you have played around in the html of your blog, adding a signature or numbered comments, removing the border round pictures etc - YOU WILL LOSE ALL THESE as the new template will overwrite all of this.
The main difference with the new Template Editor is in uploading your pictures: older blogs won't have their actual layouts or headers affected unless you change them yourself, but the Picture Uploader will default to the new one. Also, please be aware that if you have more than one blog, this will affect ALL of them.
I found I couldn't upload pictures direct from the My Pictures folder in the hard drive on my computer - this format seems to be happy only to upload via Picasa or another online site, eg: Flickr, Photobucket. Trying to upload from my computer on 6 separate occasions caused it to fail.
Note added 8th April 2010Google seem to have fixed this now, you'll be glad to know. (New programmes always have a few glitches in the beginning).
Of course, if you install Picasa on your computer, this means you can upload any image... but it also means they may become public if you are not very careful with the settings - which you might not want. (Also, it appears that Picasa has a finite size for free storage - around 1mg - more than that, and you have to pay!)
NOTE: Installing Picasa will automatically organise all your pictures into Picasa's image gallery - on your computer - where you can arrange and store all your photos in different folders (Family, Blog etc). You'll be able to upload automatically, straight from your computer, using the default setting on your blog's Image Uploader (the little picture icon) on your post toolbar with no problems whatsoever.
I am pretty sure this is a bandwidth issue: the new templates are so customisable and therefore so complex, that they must use more bandwith. Therefore, hosting your images on an external site won't use any of Blogger's bandwidth (unless it's Picasa) and your images upload like a charm. Personally speaking, I have hundreds if not thousands of images, and to have to upload a photo to an external site every time I want it to go on my blog would be a real pain - but for those of you who use external storage anyway, this won't present any problem. In fact, it would be business as normal! Just upload the image code as you normally would. It may also be worth installing Picasa, and limiting the pictures you leave in there, or upload to your blog, and keeping the rest of your images, copyright-free pix, digital scrapbooking etc, totally separate. That way you'd get the best of both worlds.
Free image storage sites, apart from Google's Picasa, are:
THE UPSIDE - HUNDRED OF POSSIBILITIES!
Install Blogger in Draft and your blog can be customised in so many ways. The possibilites are stunning!
Go to Blogger in Draft to activate.
There are four main design layouts, and under the thumbnails you'll see several variations for each design layout - so you've already got around another 20 layouts in all. I have chosen the Awesome Inc template, and the third option shown underneath.
The second step is to start customising! Click on Background in the menu on the left. The first thing you will see are suggested colourways for the template - so you could change to blues, greens etc. This varies for every single template! As you are beginning to see, there are hundreds of permutations...
Click the arrow beside the thumbnail next to the menu bar - this brings up a fly-out menu with a scrollable list on the left, showing all the categories you can choose from, with thumbnail images on the right.
I simply click on the thumbnail of the background image I like, and hey presto... OK, I'm not keen, so I'll scroll down to a different category and see if there is something else I like better.
Yes, this is much better. I'll stick with this one. Now to start customising the layout.
Click on Layout in the menu on the left, and then select the layout for the blog - how many columns, whether on the left or the right, etc. Your choice highlights in the box above, and you immediately see the result in the split screen below. Preview to see this in Real Life (I'd suggest you save the layout now, if you are happy with it).
Now to go on to customise colours and size of text etc...
Click on Advanced in the menu on the left, which brings up all the categories in the main menubar. On the left are all the text/widget/header choices with a scroll bar, and next to this is are the fonts you can choose for each section. Just below, you can choose the size you want, plus Bold and Italic.
To the right are the suggested colours that match your blog, and in the little text boxes at the top you'll notice colour codes: these start with # and are a combination of 6 number/letters. Next to these are little arrows - click on these if you want to pick a particular colour you don't see in the main section.
You can see some of my changes here. Every category you choose in the menu (eg: Post Title shows in the pic above) is outlined/highlighted with red dotted lines, so you can see where you are. Notice I've also chosen my font (Verdana) and the size.
I've also customised my post background colour, as I wasn't keen on the white. I'm happy - so I just need to click Apply to Blog, and I'm finished!
My finished customisation: I changed the background three times before I settled for this one, then I changed the layout. I changed the Blog Title colour to white, 70pt size and the Post Title to gold, 42pt size. The background for the posts has changed from white to ochre, and the Page font to 16pt (all Verdana).
There is still a way to add a customised background image, but these will need to be applied to a particular layout individually - no more swapping backgrounds daily or weekly with these new templates, I'm afraid! But if you are happy with what you see, then go for it.
If you want something totally individual to yourself - especially if you have paid for the design - then don't change to this editor - even a wide 3-col background doesn't fit comfortably although Stretch templates will work. The biggest issue for the blog designer will be applying their copyrights for any free images, as these don't position happily either. As I see it, the only way it will work properly is to download the entire package, background included, and the background will only fit that particular template.
Therefore, if you love variety in your backgrounds, EITHER go for the new template editor OR stick with the old editor and choose Minima - the most adaptable template out there. And the one which all those wonderful freebie backgrounds will fit.