Every once in a while, someone asks me about HOLDRS: the quirky, ETF-like structure that has been around since 2000.

It takes me right back to 2000, when HOLDRS were the hottest thing on Wall Street. If you wanted to buy Pharmaceuticals, the only choice was the Pharmaceutical HOLDRS (PPH). If you wanted Biotech, it was Biotech HOLDRS (BBH).

But it was more than that. HOLDRS were launched (and still trade) in what were the hottest-of-the-hot sectors at the time. If you thought the now-defunct HealthShares Ophthalmology ETF (NYSEArca: HHZ) was the first exercise in narrow-focus absurdity, try these out for size:

• Broadband HOLDRS (NYSEArca: BDH)
• B2B HOLDRS (NYSEArca: BHH)
• Internet Architecture HOLDRS (NYSEArca: IAH)
• Internet Infrastructure HOLDRS (NYSEArca: IIH)

BHH-the B2B HOLDRS-has the unfortunate title (I believe) of being the worst-performing mutual fund in history. At the peak of the Internet bubble, BHH traded for $109/share. It now trades for $0.27/share. In other words, it's lost 99.8% of its value in just over 9 years.

Which brings me to one reason I usually recommend against HOLDRS: The funds are completely static portfolios. They launched a decade ago, and that was it. If a company went bankrupt or was acquired, HOLDRS simply redistributed the weighting among the remaining components. They never rebalanced or added new names. In the case of BHH, so many "B2B" firms failed that the fund now has only two components: Ariba (91% of the fund) and Internet Capital Group (9% of the fund). And both have been awful investments.

A decade removed from their launch, most HOLDRS now have unusual weightings. The aforementioned BBH has 67% of its portfolio in two stocks: Amgen and Gilead. The Telecom HOLDRS (TTH) is 86% invested in AT&T and Verizon.

The other reason I recommend against HOLDRS is that, for all intents and purposes, they can only be traded in round lots of 100 shares. Occasionally you will see trades go through in uneven amounts, but there is no guarantee of liquidity, and you should really trade in round lots if you want to trade them at all.

But a funny thing happened when I was looking over the March ETF fund flows data: the Oil Services HOLDRS (OIH) showed up in the top 10 list, pulling in $547 million for the month. The fund now has $2 billion in assets under management.

At first I laughed: Those fools, buying HOLDRS. Surely there's a better choice.

My fears were confirmed when I looked at its holdings and saw the heavy concentrations: 17% in Transocean, 12% in Schlumberger, 9% in Diamond Offshore.

See? Too much concentration! The classic HOLDRS flaw!

But then I started looking at OIH's peers in the oil services space:

As it turns out, OIH is no more concentrated in its top names than IEZ, which is the only market-cap-weighted oil services ETF on the market today. And while some investors may favor the diversification benefits of XES and PXJ, their equal-weighting methodology offers a strong skew toward mid-caps, which may not be appealing today.

Does that mean OIH is the best choice? Not necessarily: Its weightings are idiosyncratic, and the fund only holds 16 components, far fewer than its peers.

Still, the fund is cheap: Its expense ratio is just 0.10% (technically, it charges 8 cents per share, so with shares trading near $80, it works out to a 0.10% ER). And it has good liquidity, thanks to its $1.9 billion in assets.

One other quirk of HOLDRS is that individual investors can redeem shares for the underlying holdings. If you own 100 shares of a HOLDR, you can send it in to Merrill Lynch along with a $10 fee and receive equal value in all the underlying shares in exchange. So if you like some but not all of the holdings in OIH, you could quickly alter the portfolio to fit your needs.

Is OIH the best play in oil services? I'm not completely sold. But unlike with most of the HOLDRs, there is a case to be made.