I'm sure many people on here will disagree with me, and I'm novice enough (in second year) that I might still come to different conclusions in the future -- however, my current thinking is that the HTH 6-way kit is not worth it. As I recall, it cost about 25 bucks. The only tests I think most Intex users need are pH, Chlorine, and CYA. While the HTH kit does include all of those, I still think the much better investment is in the TF100 kit. The TF100 kit will allow you to run a full battery of tests that these forum folks like to see, but even if you don't do all those tests very often, you will still get a huge bang for you buck in terms of pH, Chlorine, and CYA testing ability.
The HTH kit tests for Total Chlorine (TC), which is the sum of Free Chlorine (FC) and Combined Chlorine (CC). The HTH kit, also known as an OTO test for Chlorine, has you put pool water in a "tube," then put five drops of reagent in the tube. If there is either FC or CC present, the water will change to some shade of yellow. You will then interpret, on a color chart provided next to the tube, which color yellow it is, and your TC is the corresponding number on the comparison chart. If you just need to be sure there is chlorine in the pool, this test works. If you need to distinguish between 3 and 5, all I can say is good luck! I cannot.
In contrast, the FAS-DPD test, found in the TF100, has you test separately for FC (which is chlorine that is available to sanitize) and CC (which is "bad" chlorine that does not sanitize). It is simple to interpret and quick to perform. You put water in a vial, then spoon in a small bit of powder. If there is chlorine (FC, I think) in the pool, the powder will turn the water pink. Then drop in reagent one drop at a time (swirling between) and count the drops it takes the pink water to turn clear. Very easy to see when it goes from pale pink to clear. No guessing. If it took 8 drops to get clear, your FC is 4. Then to test for CC, add 5 drops of another reagent and if it turns any shade of pink, add in another solution one drop at a time to get it clear again. Count the drops to get it clear, divide by two, and that is your CC. Knowing your FC and CC with precision is very empowering, as it tells you when you have sanitary water and when you need to shock or otherwise address issues relating to low chlorine. I do my FC, CC, and pH testing, start to finish, in less than 10 minutes.
I can't recall the number of CYA tests you'll get out of the TF100, but I think it is 6 or 7. You get 2, maybe 3 from the HTH kit. For me, because I have a lot of splash out from my small Intex, I tend to want to test CYA once every three or four weeks to be sure I know my correct FC minimum and maximum. Again, I know I can't clear an algae outbreak like pool owners who have powerful pumps and filters so it is very important to be proactive and not assume I can react if algae breaks out. Keep in mind, dumping and refilling is not a great option for me but it might be more palatable to others, in which case the HTH might be enough since they are more willing to gamble with their original pool water.
As far as I know the pH test is roughly the same between the HTH and the TF100. I still use my HTH for pH.
In my opinion, the key to an Intex is making sure nothing ever starts to go bad. Thus, I always felt like if my minimum FC should be 4, for example, I better not mess up and interpret my OTO test as being the subtle shade of yellow assigned to 4 when in fact it was a 3. Seemed to me like I was gambling with my kids' ability to enjoy the pool on a sunny day, so I use the FAS-DPD everyday.
In short, the HTH kit certainly works for some people but in my opinion if you can afford the FAS-DPD, it is money extremely well spent. For that matter, I also love the magnetic stirrer thing that TF Test Kits also sells, for about $34. An Intex, plus $120 or so the first year ($68 original TF100 +$ + shipping) and less in years to come (refilling the reagents is much less $) seems to me to be a sound investment to have the best chance of keeping the pool sparkly all year.
By way of full disclosure, let me tell you I don't have a TF100, but that is because last year it seemed more cost efficient to me to separately buy the FAS-DPD kit ($25ish) and the CYA kit ($17ish I think) since I already had a 6-way kit including a pH test from HTH. So I bought the individual kits from duraleigh, who sells the TF100. THIS year, however, the TF100 kit has double the reagents from last year, so for $68 you can get what I bought as the FAS-DPD individual kit, about 1/2 the CYA individual kit, plus all the other testing stuff including pH and all the vials, blocks, and reagents and powders. (note, duraleigh or someone might have a more precise comparison in terms of the TF100 versus his individual kits, but this is the way I eye-balled it). If you become a TFP Lifetime Member before you order your kit, you can get that huge kit for only $48 the first time. Awesome deal!!