(This post has been edited to correct chemistry errors :0)
There are Baqua-Spa lovers and haters the world round. Baqua-Spa is a Biguanide based alternative sanitizer (Baqua-Shock/Oxidizer is Hydrogen Peroxide based) - (to bromine or chlorine)... it's good stuff, it works for many. But the unfortunate thing is the corporate recommended regimen that goes along with it, (as well any particular manufacturers regimen). I'm not against Baqua-Spa, and many spa owners I've met actually use their program religiously, and I'm not in a position to disagree with them. My thing - I look at the leakage traces from the pump seals, oxidation at the heater element connections etc, consider the age of the equipment, length of time of ownership etc.. to see if what they've been doing, has been successful or a failure.
By far, the majority of people that use the Baqua-Spa program religiously, are experts in their hot tub environment, their bather load, and other things and they pay attention to details, and know what they're talking about when it comes to managing the water quality in their tub. But it does take some self education to get right -
http://www.baquaspa.com/Docs/BAQUASpa_Guide.pdf This is not easy reading, but if you're looking for a very high quality result, and you're a discriminating individual or family, then it might help if you spent some time to learn the process. Your spa dealer is not to blame really (he's got his own personal tried and true formula that works for him and his family) - everybody wants a hot tub like yesterday, and everybody wants it to be trouble free tomorrow, but water quality management is a learned process and eventually becomes a personal skillset that will stay with you for life if you stick with it.
There is no one single formula for a hot tub. Water quality management for a spa in Florida, used by 1 person, will be entirely different from someone in California - and water quality for a family of four that uses the tub weekly will be vastly different from anyone from a 2 person household located in anywhere usa.
Now let's address a couple of things:
1. Weird brown goo ring. That's usually because of a plasticizer (clarifier) that's been added to the spa, to eliminate cloudy water. This is quite normal. I was in one of my (customers) house today and demonstrated how quickly adding a 'clarifier' to the water would start producing crud on the water line. Just wipe it off. Then again, if he's sticking around for a bit afterwards (after adding the clarifier), squirt a bit of de-foamer on the brown accumulating bubble areas and they'll disappear - for now, and help to eliminate the goo ring. (Instead of collecting on the side walls, they'll end up coating the filter cartridge, which must be cleaned eventually). No matter what - Crud is in the water, and you want to get it out so it looks nice. Clarifiers are great - use them VERY sparingly. The more you put in, the more goo you get on the walls. Baqua Spa can also add to this goo.
2. Bitter taste - For one thing, I would NEVER taste hot tub water as easily as I would one from a well maintained swimming pool with adequate sanitizers. Pools are generally cooler in temperature, and regulate slowly because of the water volume. Hot Tubs on the other hand, can be contaminated (like max 400 gallons) in minutes - and can propogate some really bad stuff pretty quick - think diarreah leaking :-[ in the tub or alternatives, because of the lower volume of water, higher temperature which helps bacterial things grow FAST. I'd rather swallow a gallon of public swimming pool water to an ounce of any hot tub water.
3. Wierd Smell - If you've never owned a spa or pool before, then you'll eventually get used to the normal scents of chemical oxidation byproducts of bromine, chlorine, or in your case Baqua-Spa. You may also have residual smells of the tub's manufacture from the cements used in the plumbing - this is normal.
I wouldn't deep six the seller, or the method of sanitization just yet (based on your initial impression), but I would give this recommended regimen a little more time to and work to make it work for you before changing your entire methodology of hot tub ownership.
At any rate, if you wanted to convert to a simple bromine plan, (If you put too much Bromine it'll stink just like the previous sanitizer), simply drain the tub, refill, and start over. There's not enough of the previous sanitizer left in the typical tub to make a difference in a refill anyway.
If it were my tub, I'd just go slowly, take test measurements regularly, and pay attention to the results, then react accordingly. Just don't freak out - this is a learning process, it's like you're trying to keep a bathtub full of water for 3-5 months at a time, and make it look and feel good while doing it. Every situation, locale, homeowner, household size and usage will require different solutions, and every single one of them is OJT.
Hope this helps!
Cheers.
(Thanks for the correction chem-geek).