I've got two characters with Herbalism/Alchemy combo (around skill 225 now, just got Artisan skill... 5 gold, ouch!) and they have both had great benefits from it. The buffs really increase your survivability and ability to keep grinding without downtime. If doing elite quests (or just mobs way above our level) with a guild group I usually hand out defence and regen potions etc which take some healing burden off the priest (me!) and thus reduce aggro control issues.
Whenever I'm doing quests in a specific area I usually end up with a surplus (ie 60+) of the common herb for that area, for example Kingsblood/Liferoot/Stranglekelp for the Wetlands. These sell well on the Auction House, bringing in gold with no production cost at all (ie 100% profit once the training cost has been recovered), presumably there's Alchemists who do not do Herbalism or just want to get the herbs at once without the hassle/time of gathering. However I have not seen low-end herbs such as Silverleaf and Peacebloom sell well, probably since they are trivially easy to get hold of.
I also make some potions for sale, however this requires factoring in the cost of vials and multiple components. Low-end potions do not seem to sell well, mid-level I've had a few returns but made some gold from selling Shadow Protection, Defence and Water Breathing. Try to figure out your market audience, if there's a lot of guilds running raids or instances during the weekends they may snap up all available potions to boost their success if you start auctions Thursday night and re-list as necessary whereas Sunday night to Thursday might be 'dead' time for getting good sales.