No matter what brand or model... make sure you can easily thread it. Mine is quite vintage and takes me a minimum of 1/2 hour to thread... many times it takes longer. Self thread it great but when the tube for the tread gets a clog ( and it will) you are out of business till it comes back from the repairman.

When I service sergers, I know I can always get a quality stitch from a Bernina. But, that's not what everyone wants. Some don't want to learn how to thread their serger. If you thread your serger about 5 times in a row, you should have it, and it's not that big of a deal. I own a Bernina 800 DL, and have never had any problems with it. I run a business from home, sewing 4 layers of Jersey knit, which my old White couldn't do. Then I switch from Jersey knit to fleece, to French terry, to regular cottons, without effort. I occasionally tweak the tensions a bit, or adjust the cutter width. It has a beautiful built-in rolled hem, and differential feed. If you want a workhorse machine, a Bernina is a great way to go.

If you can afford the $3000+ price of the Babylock with air threader, you'd better have a Babylock dealer nearby, because when the air threader goes out, you can't thread it by hand. You'll have to take it to the dealer to have it fixed. We won't touch the threader at the repair shop where I work, because we're not trained on them.

I have a 13 year old Bernina that I don't think I could sew without. I wanted a serger that did a cover stitch; but haven't used a cover stitch in years. The overlock and rolled hem stitches, I use constantly. While I think my serger is the greatest, the manual is lousy. Hopefully, now their serger manuals are better written.

I have a brother 1034D which I am quite pleased with. It is not a self threaded but I don't change the thread all that often and there is a video that comes with it plus some you tube videos to help with threading if you can't tie your new thread to the old and pull it through. I wasn't willing to spend thousands for the air threading. I don't use it for quilts though.

I have an older Pfaff. I have looked at the newer sergers, all brands. I decided it wasn't worth the $$$ to pay more just to get the self threading model. Threading is easily learned. Of course, I rewrote the instructions and color coded them so that I could understand it all. Remember I translate most directions. I also taught math in a past life.

I have a babylock i got when my oldest was little. 1989? maybe. used it for his and his brothers baby clothes. it comes in handy for alot of things. the rolled hem is good. its not hard to thread. there is a learning curve. now i want to use it in quilts some too. (its great for compressing the edges for binding. Kaye Wood does a serger quilt.

So agree with thinking about how you will use the serger before you make a purchase. If you want to hem tee shirts (some of us are not 6 foot tall, and those overly long tee shirts look ridiculous on us!) you will need one with a cover stitch. Only a few sergers offer this. If you think you would use this feature a lot, you might buy a cover stitch machine -- and keep it set up just for this use.
Let's talk price -- if you are willing to learn the machine and to thread it yourself, you will save a bunch of money. If money is not an issue, the Baby lock self-threading machines are excellent - their self adjusting tension, and all the various functions are well worth the price... and there is a substantial difference in price -- only the top of the line offer the cover stitch. This is not a machine you would only want to use once or twice a year. If you have a local store which supports the Baby Lock line, you might want to see what serger classes they offer - using any serger! And there's a good class on Craftsy too -- often offered at a discount. I took it and learned alot.
PS: I do own a Husquvarna Viking S21. Learned to thread it myself with the computer disc tutorial. I use it in spurts -- 3-4 times a year for shortening things (it has the cover stitch), and am learning to sew tank tops and other knits items from scratch. The threading definitely had a learning curve. The only disappointment I've had with this machine is the tension adjustments are barely noticeable, but have managed to make it work for all my needs. My local LQS offers serger classes several times a year -- even using your serger to do quilts -- they can be quick and easy and very beautiful.

I had a Phaff serger and never used it because it was so hard to thread. I sold it and bought a Babylock with the air threading. Love it. Would haven't any other kind. It's do easy to use. Check it out!

Even though Babylock is the only one with the pattened air threading there are lots out there that are easily threaded. Once you have them threaded then you cut the thread at the spool, then tie it to the next spool and pull it thru. I also have the Viking Huskylock 21 and its color coded and easy to thread and has lots of features along with self adjusting tensions like my emb machine has. I love this feature. I was lucky to be able to get the Babylock with air threading and no tensions settings but don't rule out sergers just because you can't afford or willing to pay the big bucks. There are lots of tutorials on line that helps see what all you can do too.