Appearance and Construction
The Parker Reflex is a slender, plastic fountain pen (FP) with a textured rubber grip and a metal spring-loaded clip. It's 5.5" long capped, and about 6" posted. The barrel is 7/16" wide at its widest point. The cap, which is 1/2" wide at its widest point, snaps onto the barrel. The Reflex is available in several colors; mine is black. It looks and feels like a cheap pen --- unfortunately my photo below makes it look better than in real life.


The Parker Frontier is a slender FP with a smooth rubber grip and a metal clip. The cap is stainless steel, and the barrel is available in plastic or stainless steel. It's almost 5.25" long capped and about 5.75" posted. The barrel and cap are about 7/16" wide at their widest points. The cap securely snaps onto the barrel. It was difficult to remove the cap at first, but with use it's gotten a little easier/looser. The threads in the gripping section and barrel are plastic (even for the all-stainless model).
One of my Frontiers is new-old-stock (NOS), with a marbled red and black plastic barrel. Naturally it's more colorful than the all-stainless Frontier, though up-close the "marbling" effect looks a little cheaply done.



All three pens post securely, though the plastic barrel of the NOS Frontier has developed light scuff marks from posting.
My stainless steel Frontier came with a hard dark-grey plastic case (see photo below). It can house two Frontier pens, and there's a little exposed compartment on the bottom that can accommodate two ink cartridges. It's a little tricky to close the case shut --- I have to press on the entire upper half of the case to close it. I'm not sure, but I think only Frontiers sold in Europe include this useful plastic case. Those sold in the US are in a black and white gift case.

For Parker enthusiasts, the date code on my NOS Frontier is IIIP, and on the all-stainless is IE. Both Frontiers were made in the US. The Reflex was made in the UK.
Filling System
The Reflex and Frontier FPs use a cartridge/converter system. My Reflex and stainless Frontier did not include a converter, only an ink cartridge, while my NOS Frontier gift set included Parker's standard converter and several ink cartridges.



Nib Design and Performance
All of the pens have stiff nibs which aren't very expressive. I guess that's pretty standard in this price range.
The Reflex has a slightly hooded stainless steel nib. The nib and the visible part of the feed appear to be of the same design as the more expensive Parker 88 and Rialto. The nib is quite modest, so if you prefer flashy nibs, this pen isn't for you.
My Reflex has a medium nib. It's a very wet writer when filled with the included Parker ink cartridge. The nib is quite smooth, with some feedback from the paper.

The Frontier has an attractive, fully exposed stainless steel nib. My Frontiers are gold-plated; other Frontier models are not plated. Although both are labeled as medium width nibs, my NOS Frontier writes a smooth, fairly wet, fine line with just a hint of scratchiness at certain angles. On the other hand, my stainless Frontier has an ultra smooth, super wet nib --- it's like writing on butter. It's so smooth that people, who like a little bit of feedback from the paper, probably won't like it. But if you rate a FP on smoothness alone, you'd love my all-stainless Frontier.

Since both the Reflex and Frontier are mass-produced, there's probably some variability in the characteristics of each nib. So your mileage may vary. Some postings on FPN hypothesize the nibs on the all-stainless steel models get some extra attention. My very small sample size appears to support that hypothesis.
Both Frontiers were tested with Sheaffer Skrip black ink, using Parker converters.
All 3 pens are well balanced, both posted and unposted. Naturally the all-stainless Frontier is the heaviest of the three, but it's still quite light. The Reflex is so light that it feels cheap. The rubber gripping sections make it easy to hold all 3 pens securely while writing.
Cost and Value
I bought the Reflex at a local supermarket for about US$7. I obtained both Frontiers on eBay; the NOS gift set for $9.99 plus USPS shipping, and the stainless steel for 2.21 GBP (about US$4.43) plus shipping from the UK to the US.
Conclusion
I think the Frontier easily outperforms the Reflex; the Frontier looks better, writes better, and has a classier feel to it, for about the same price as a Reflex. Unfortunately it seems most of the Frontiers are sold in Europe (at least that's my impression), so those in the US will have to pay extra for shipping.
The Frontier has a conservative look that'll fit right-in at the office. If you want something flashier, the Waterman Phileas is a nice alternative that writes as nicely as a Frontier, for a bit more money.
All-Stainless Steel Frontier: excellent
Plastic Barrel Frontier: very good
Reflex: poor


