You should try a tomato directly off the vine if you haven't already. I used to hate tomatoes but after tasting a tomato grown in my friends garden, I realized that I had never actually tasted what tomatoes are supposed to taste like. Tomatoes from the store and restaurants are all watery and old by the time you eat them.
I've noticed in going through the tree that estas (present tense to be) is interchangeable with the -as ending which can be used to change an adjective into a present tense verb.
For example; 'Glacio estas malvarma' and 'Glacio malvarmas' both mean ice is cold.
You don't us estas when -as is being used and vice versa and in some sentences no mater what -as is more appropriate.
Consider the sentence 'Mi legas la libron' or 'I read the book'; you don't say 'mi estas lega la libron'. Google translate says lega is bequeath but I don't trust it but that might translate as 'I am bequeath the book' maybe? Either way not grammatically correct.
I think your sentence would translate as "Tomatoes are a very bad taste'.
Maybe one of the experts can correct me on this?
Two years later ...
The information I supplied in my previous comment is not true. The verb form of an adjective or noun is NOT synonymous or interchangeable with "to be adj/noun".
The verb form means "to perform the action associated with the root". Just like the English "to hammer" means "to use a tool in a hammer-like way to pound an object".
Therefore, "glacio malvarmas" means something closer to "ice gives off coldness". Not all verbifications are clear, and some are best left to poetry.
Both "estas tre malbongusta" and "tre malbongustas" mean the same thing.
In Esperanto, you can always verb an adjective, and it means "to be [adjective]".
As you said, "glacio estas malvarma" is the same as "glacio malvarmas".
So "estas malbongusta" and "malbongustas" both mean "are untasty" or "taste bad".