Hi Hawking, sorry I didn't see this thread earlier.
I shoot my Tar-21 in both CQB and Service Rifle. While an insanely overpriced 5.56mm rifle, it is a lot of fun and gets many a strange look from people (and the 'oh my god, bull-pup's are illegal' crowd). I'll shoot you a PM, with luck, we should both be at the Clinics this year.
Mag changes are stupid fast when your good at it, rare for me though... damn Army trained me to do changes on the C7... magwell in wrong spot lol (muscle memory always wins when under stress and I constantly try to stuff the mag into the front fore grip when not focusing lol). However a good AR guy with the B.A.D. installed can really fly now.
As people have said, you simply use your trigger hand thumb to come back and drop the mag, other hand seats new mag and the thumb on that hand hits the bolt catch. Its insanely smooth when you can get it (non-AR shooters seem to do a better job when playing with my Tavor since they don't have the muscle memory).
You'll get used to the Tavor mark soon enough, nice black powder marks under your shooting glasses. Just remember to check the mirrors in your car before driving off or people will think you were playing football.
Slings are a pain for this rifle but a simple hunk of paracord made into a loop in the rear hole of the buttstock lets you use a carabiner to suspect the rifle from your tacvest shoulder loops (if you use the Can. TV).
Stick with standard mags for now, P and E Mags gave me loads of trouble initially (though I'm sure someone has figured out the issue) and you don't want sticky mags in CQB.
If you replace the Mepro sight, install one of many of the rail options out there (I prefer the smaller rail that mounts in place of the Mepro) but many of the Tavors now come with a huge full length top rail (looks too AR-Magpul-Lego-Etc for me). Most sights with this smaller rail will co-witness just fine but no idea about the newer flip-up and full rails.
Here is the small rail:
http://www.canadaammo.com/product.php?productid=61&cat=0&page=1
You can also order a quick-release replacement mount for the Mepro, check around I've posted the info somewhere on here. I think the mount was around $99.
Careful with Google searches, most of the hits will be for Airsoft items... though sometimes you can find deals on real items that for some reason are massively marked down for Airsofters (manufacturer or distributer just puts it in a different bag, guess Airsofters are on a lower budget). Especially keep your eye out for the MARS sights that randomly seem to come on sale, I've missed them each time but they are the original sights for this rifle and you can pickup the fore grip button and safety power off/on assembly for them.
They don't seem to come with bayonet lugs, no idea why (likely contributes to the non-restricted though) but then again, who needs it. Still love to toss one on for "completeness sakes".
Get used to Zahal.org, great site with toys for the Tavor and other IDF items. Buy one of the fore grip under rails if you want so you can use grips like the TFL (it's small enough that it folds on the Tavor). Nice T-Shirts too
http://www.zahal.org/products/tavor-picatinny-rail
http://www.zahal.org/products/tactical-folding-grip-tfl?path_parent=218987
Grab the armorers manual in PDF form, links should be in the Tavor faq.
Hold off buying any scopes if you haven't bought one till the clinics, that way you'll get to try TONS of different models and see what you like. I settled on the Trijicon Acog TA11F as the eye relief was amazing (looks like HDTV lol).
It is a paid in the ass to field strip/clean, can't take it full apart as easily as an AR (to be able to get at the chamber) but in a detail strip, you can take the entire barrel out and then it's a dream. Just be very careful when putting back together, there is a trick to the barrel lock and if you have to really force it, something is wrong.
Everyone complains about the trigger but it doesn't both me so far (anything is better than beat up C7's I suppose). Modifications are our there (remove a spring) if you want to try.
AR Pistol mags (LAR-15) work just fine but due to the depth of the magwell, I'd recommend installing magpul style plates on the bottom (not the rubber ones, the ones that replace the mag bottom plate). Did it with mine and they fit nicely into the grenade pockets on the TV to boot.
Cheers
Tim