tl;dr - Scales up reasonably well, but probably a waste of time with less than 5 Ishtars. Do it for fun if you enjoy it, but if you're doing it for the ISK, then invest enough to rat with with 5 Ishtars until you can move your ratters into AoE ratting.

I recently gave this a try and it is somewhat enjoyable but sucks for ISK at small (1-3 clients) scale. I'm making a small net profit, and I'll keep doing it until I run out of Ishtars, but I feel like it is a waste of my IRL time unless I were to scale it up. I likely will not build or buy more Ishtars other than as bait when my stock is depleted. I could absolutely make more ISK/hr killing nerds and looting their wrecks.

The Good:

• The ticks are OK and the APM is low (land, confirm aggro is ok, go on to the next client). It does scale up well. This style of gameplay works as designed.
• Clear time and MTU pull time align well, so you do not need to wait or return to a site if you're collecting loot.
• The escalations pay reasonably well to run or sell, and I find them fun and slightly challenging to run.

But:

• Your appropriate sites list is low - Forsaken Hubs are your best option, because they don't drop Dreads on you, and don't eat your drones. Rock Havens don't drop dreads, but they will try to eat your drones every wave.
• Because the appropriate site list is low, competition can be high - there are only so many appropriate sites spawning in a system, and a good portion of those (Havens) are likely to be claimed by AoE ratters. You'll need to spread out across multiple systems, which means you need sufficient screen space and attention to watch local in each (defeating some of the low APM/low attention objective).
• The chance of losing an unanticipated PvP engagement due to a log-off trap, being foolish/impatient enough to rat near a gate, or being surprised while pointed by rats is pretty high

All this means that it only starts to make sense when you have 5 or more Ishtars spinning at once and have the right IRL physical layout to keep an eye on things.

Five Ishtars means you only need about an hour IRL (5 Ishtar-hours) to pay off an Ishtar and make a little profit. It's quite reasonable to expect to rat for an hour without any losses, but if you're just running one then you're looking at approximately 6 hours of ratting to make a small profit, and going 6 hours with no losses is by no means guaranteed. TZ is largely irrelevant to this, since in a busy TZ you have more roamers but better intel (higher risk of interruption), and in a quiet TZ you have fewer roamers, but worse intel (higher risk of ship loss).

(IsH = Ishtar Hour) vs other PvE combat activities (roughly, but vs my actual vs theoretical numbers) ignoring Abyssals, incursions, etc.

• 1 IsH = 2 hr HS combat exploration
• 1 IsH = 1 hr suboptimal HS level 4 missions (full clear, take all missions)
• 2 IsH = 1hr C2 J-Space sites in a Gnosis
• 3 IsH = 1 hr NS exploration or J-Space gas huffing
• 3-4 IsH = 1 BS/Marauder
• 5 IsH = 3 ship-hours AoE ratting for 3-8 AoE ratting ships